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From A to Z

Our glossary contains most of the technical terms used in the world of aluminium oxides and solids. It does not claim to be complete and will be updated by us from time to time. If you have any questions, please do not hesitate to contact us.




Wearing down or wearing away of a massive solid by rubbing or friction; often deliberate. See also Erosion, Attrition.


A granular or particulate solid used with mechanical action to abrade or remove another material, e.g., by grinding, lapping, honing, or polishing. Grain or grit size, hardness, and toughness are important.



The taking up of a liquid or gas by capillary, osmotic, or solvent action. See also Sorption, Adsorption, Chemisorption, Physisorption.



Refractories Refractories which are reactive toward (i.e., corroded by) basic melts, slags, and fluxes, and resistant to acidic melts, slags, and fluxes. Examples are silica and relatively high-silica-containing refractories, including those of mullite and clay-alumina mixtures.



A process of developing optimum chemical or surface reactivity in a solid material, usually by appropriate heating. See also Calcination. 


Active Alumina, Activated Alumina

Any of several synthetic high-surface-area transition-alumina products made by proprietary activation processes; available in various convenient forms and modifications. Chi/rho/eta alumina. 


A substance distributed in another material in relatively small amounts to improve desirable properties or suppress undesirable ones.



The taking up of a liquid or gas or of a dissolved substance, only one to a few molecular layers in thickness, on a solid surface; held there by molecular forces. Often subdivided into chemisorption and physisorption. See also Absorption, Sorption, Sorbate, Desiccant. 



The clustering together of a few to many fine particles or crystals into a larger solid mass; also, the resultant assemblage of fine particles or crystals, which may itself be a particle, or larger.


The hard, dense, coarse granular material such as sand, gravel, periclase, or tabular alumina, etc. with which a cementing or bonding material is mixed to form a mortar, concrete, brick, block, or other strong heterogeneous mass by setting, curing, or firing.


Alpha Alumina

The highest-temperature, hardest, most chemically stable, and inert crystalline form of alumina, Al₂O₃; found in nature as "corundum," or made synthetically by intense heating of alumina hydrates or other aluminas, usually to temperatures from 900 °C to even above the melting point (2050 °C).



Aluminum oxide, Al₂O₃, in any of its many forms. As a general term, includes the lowertemperature transition-alumina forms (containing perceptible hydroxide as well as oxide anions) and may even include alumina hydrates and a few related compounds such as dawsonite. See also Alpha Alumina, Beta Alumina. 


Alumina Hydrates

See Aluminum Hydroxide, Alumina Trihydrate, Gibbsite; also, Alumina Monohydrate, Boehm-ite. A general term most often used for the trihydrate. 


Alumina Monohydrate

The oxyhydroxide of aluminum, AIO(OH), also written Al₂O₃ H₂O, in any of several forms. Occurs in nature as a constituent of bauxite; also made synthetically by any of several aqueous reactions or by hydrolysis of certain aluminum-organic compounds. See Boehmite, Pseudoboehmite, Gel Alumina, Substrate Alumina; also Diaspore. 


Alumina Trihydrate

The true hydroxide of aluminum, AI(OH)₃, also written Al₃O₃•3H₂O, in any of several crystal-line forms. Occurs in nature as a constituent of bauxite; also made synthetically by the Bayer process. See Gibbs-ite, Bayer Hydrate, Bayerite, Nordstrandite.


Aluminum Hydroxide

Properly, aluminum trihydroxide or alumina trihydrates; loosely, a general term for any of the alumina hydrates.



Lacking a detectable crystallinity; liquid-like (i.e., disordered) in its structure, though solid in appear-ance. A loose term, dependent on the tools used and dimensions allowed in identifying the regular ordering of atoms. See also Glassy. Often, lacking discrete X-ray diffraction peaks.



A unit of length: one-tenth nanometer (1Å = 0.1 nm = 10⁻¹⁰ m). About equal to the diameter of a single atom of hydrogen.




The agglomeration of particles into large spheroidal masses during high-temperature rotary calcining, usually due to melting of a minor constituent or sintering of a major one. Also, see Nodulizing; Clinker.


Ball Mill

A cylindrical or conical shell rotating on a nearhorizontal axis and charged with balls of a grinding medium substantially harder and larger than the material to be ground. Used for fine grinding as opposed to crushing.



A mineral or ore containing one or more alumina hydrates in conjunction with other oxidic compounds of aluminum, silicon, iron, etc. The principal source of synthetic aluminas, aluminum, and other derivatives via the Bayer process.


Bayer Hydrate

Gibbsite, a particular crystalline form of alumina trihydrate, Al(OH)₃; the primary product of the Bayer Process.


Bayer Process

The near-universal commercial process for producing alumina (and hence aluminum) from baux-ite. Consists of digestion in hot caustic to yield a solution of sodium aluminate; separation of undissolved solid wastes ("red mud"); and precipitation of gibbsite, Al(OH)₃, from the liquor by seeding and cooling, followed by filtration and washing.



Alumina trihydrate of a different crystalline form from gibbsite; rare. Not related to the Bayer process.

BET (Brunauer-Emmett-Teller)

An instrumental method for determining surface area of a solid sample, measur-ing monomolecular nitrogen gas adsorption at the temperature of liquid nitrogen ( -210°C). Some other sorbates are also used.


Beta Alumina

Properly, not an alumina but one of a family of alkali metal polyaluminates, e.g., Na₂O•11Al₂O₃. Often a minor impurity in fused alumina; also, a synthetic ceramic material.



The commonest crystalline form of alumina monohydrate, AlO(OH). A constituent of bauxite; also, an important synthetic: the major component of commercial gel-derived alumina, also called substrate alumina. See also Gamma Alumina, a derivative. Pseudoboehmite is ultrafine-crystalline.


Bulk Density

See Density.


Bulk Specific Gravity

See Specific Gravity. 


Bulk Volume

The volume of the container occupied by a quantity of unconsolidated solid material in the form of particles, grains, agglomerates, nodules, pellets, etc. Must be given other descriptors, as (*loose," "packed," and the manner of packing. See also Nominal Volume. As applied to massive consolidated materials or objects, the bulk volume and the nominal volume are the same. See Density.



Calcine, Calcination

Firing or heating a granular or particulate solid at less than fusion temperature, but sufficient to remove most of its chemically combined volatile matter (e.g., H₂O, CO₂) and otherwise to develop the desired properties for use. Also importantly, the product of such a process, e.g., "calcine" "calcined alumina," available in numerous modifications for numerous uses. See Burn,; also Degree of Calcination.



A chemically active agent which functions to increase the rate of some chemical reaction without itself being appreciably consumed or altered. Often such agents are solids (heterogeneous catalysts), distributed as very thin layers over the immense surface area of a suitable porous solid substrate.


Ceramic, Ceramics

Any of a broad class of inorganic, nonmetallic materials or products which are subjected to temperatures above about 540°C or 1000°F in either manufacture or use; by incorporation, many others of similar constitution or characteristics which are not necessarily so heated.


Ceramic Fiber

A ceramic material in fibrous or filamentary form, whether glassy, polycrystalline, or single crystal in structure; made by, e.g., blowing, spinning, drawing, whisker growth, etc. Useful as fillers or reinforcement, as fabrics and insulating felts, in optics, etc.


Chemical Properties

Those intrinsic characteristics of any substance, material, mixture, or article made therefrom which relate to its chemical interactions, reactions, changes; either internal, on contact with others, or with components of its environment. Loosely, includes the chemical analysis or composition itself, often with phasis on impurity content. 



The taking up of a liquid or gas or of a dissolved substance, only one molecular layer in thickness, wherein a new chemical compound or bond is formed between the sorbent surface atoms and those of the sorbate. Reversible with difficulty. See also Adsorption, Absorption, Sorption, Physisorption. 



Vitreous, translucent white porcelain ware for domestic and ornamental use, as distinguished from industrial whitewares and other less-fine domestic ceramic wares. Usually glazed, esp. for decorative purposes.



Any of a broad class of natural mineral agglomerates of very fine, usually platelike particles of hydrous compositions including combined alumina and silica. Often plastic when wetted, somewhat cementitious on drying, and vitrified when fired. Used in making numerous ceramics and some refractories. 



Refractories Refractories mainly and/or prised of various clays, bauxite clays, bauxite, and/or synthetic alumina, selectively or together in prescribed proportions. In order of increasing Al₂O₃, and decreasing SiO₂ content, these range from acidic to nearneutral and generally increase in refractoriness.


Closed Porosity

The volume fraction of all pores within a solid mass that are closed off by surrounding dense solid, hence inaccessible to each other and to the external surface; thus not detectible by gas or liquid penetration. See also Porosity, Connected Porosity. 


A relatively stable suspension of some material within a fluid host, the dimensions of the former usually being about 1 µm or less. Fogs, smokes, foams, emulsions, sols, and gels are examples. 

Colloid Mill

High-shear mill designed to grind particles down to colloidal size, i.e., to the order of 1 µm or less.



A heterogeneous material, body, or article comprised intimately of two or more widely different material classes: metal-ceramic, ceramic-organic, glasscrystalline, etc.; ordinarily combined for some synergistic effect on performance. Components may be layered, fibrous, particulate, etc. 


Conductivity (thermal, electrical)

The ability of a material to conduct or transmit heat or electric current, measured by standard means.


Contact Angle

Angle formed at the edge of a liquid on a solid surface, the resultant of the several interfacial tensions at work. Low angles characterize ''wetting'' high or negative angles increasingly "nonwetting."



The eating away of the surface layers of a material by an external chemical agent, e.g., of iron by moist air or of refractories by melts, slags, and fluxes. See also Stress Corrosion. 



See Alpha Alumina.



The compound Na₃AlF₆; used as a flux, expecially in aluminum smelting. 



That form or particle or piece of a substance in which its atoms are distributed in one specific orderly geometrical array, called lattice, essentially throughout. Crystals exhibit characteristic optical and other properties and growth or cleavage planes, in characteristic orientations. See Diffraction. 



That form of a substance which is comprised predominantly of (one or more) crystals, as opposed to glassy or amorphous solids and liquids. 



A small or tiny crystal; or each individual crystalline domain in a polycrystalline solid mass, differing from its neighbors at least in orientation. See Grain.



The progressive process in which crystals first nucleated (started) then grown in size within a host medium which supplies their atoms. The host may be gas, liquid, glass, or of another crystalline form.



A cylindrical-conical vessel, entered tangentially by a gas stream such that the internal gas path is circular at high velocity. Used to separate entrained particulate solids from gases by centrifugal action.




Without modifier, the weight of an object or material divided by the volume it occupies (its external volume), hence its weight per unit volume; ordinarily expressed either in grams per cubic centimeter (g/cc or g/cm³) or in pounds per cubic foot (lb/ft³ or pcf). The weight of an object or material is readily determined. However, in materials handling and usage several characteristic "volumes" and their corresponding void fractions have to be recognized; each "volume" gives rise to a different characteristic density:

  • Theoretical Density ("True" Density) Computed from the theoretical volume of the solid as measured, for example, by X-ray diffraction. This characteristic volume contains no pores or voids. Use of the word "true" is discouraged.

  • Apparent Density Computed from the apparent volume, which is the theoretical volume plus the volume of closed pores only; measured, for example, by determining the nominal or envelope volume and subtracting from it the water absorption or mercury porosity (approximating the connected or open pore volume). For nodules, pellets, and extrudates used as desiccants and catalysts, the apparent density is also called the skeletal density.

  • Particle Density Computed from the nominal or envelope volume. This characteristic volume includes the theoretical volume plus that of all internal pores, both closed and open. It can be measured, e.g., by pycnometer after wax-coating the particles or object.

  • Bulk Density Computed from the bulk volume of an unconsolidated solid. This is the overall volume (viz., volume of container occupied, now including also the interparticle packing space. The manner of filling (e.g., "loose," "packed," "tap," "tamped," or "vibrated," and details) must be specified as well. For a consolidated solid, the bulk volume and nominal volume are the same, and the bulk density computed therefrom accounts for the total porosity. Consolidated materials may require further process descriptors prefixed to density as well, e.g., ''green," "dried," "cured," "fired."


A material used to dry another or to dry the atmosphere, by virtue of its ability to sorb water, e.g., active alumina, zeolite, silica gel, etc. See Sorption, Adsorption, Drying. 



The reversal of any of the processes of sorption: the removal or reevaporation of a sorbed substance. See Adsorption, Absorption, Chemisorption.



A particular crystalline form of alumina monohydrate, AlO(OH), occurring as a mineral usually bonded by clay. See also Boehmite.



A nonconductor of direct electric current, an insulator material; also, one which exhibits the phenomenon of polarization (internal molecular reorientation under an electric field), useful in capacitors, hence in electronics.


Dielectric Strength

A measure of the ability of a dielectric (insulator) to withstand a potential difference across it without electric discharge.


Differential Thermal Analysis (DTA)

An instrumental method of tracing the temperature of a sample while its container is heated at a constant rate; discloses endothermic and exothermic processes occurring in the sample, e.g., decomposition, phase changes, combustion, etc. See also Thermogravimetric Analysis. 


Diffraction (X-ray or electron)

An optical phenomenon and instrumental method whereby crystals can be identified and their atomic arrays and dimensions accurately determined; also provides the theoretical density of each crystalline substance, and can estimate the size of crystals up to a few hundred angstroms. 



The relatively slow movement of one or more kinds of atoms, molecules, or ions through a host material, in consequence of concentration or temperature gradients. Also, especially of a gas, through the connected porosity of a solid; see also Permeation, Knudsen Diffusion.




The mobile-ion-containing medium (ionic conductor) in an electrochemical cell. Most often, an aqueous solution or a molten mass or "bath" of inorganic compound(s); more rarely, a solid ionic conductor. By extension, any such "ionic" material system whether used electrochemically or not.


Electrostatic Precipitator (ESP)

A device used to separate very fine solid particles from an entraining gas, by inducing a charge on the particles and collecting them by attraction on a surface of opposite charge.



In chemical or physical changes, one which absorbs energy or heat and thus tends to cool the medium in which it occurs. See also Exothermic.



The process of grinding or wearing away of a massive solid surface by rubbing or friction, generally by impact of particles of some other material. Usually undesirable. See also Abrasion, Attrition, Corrosion.


In chemical or chemical or physical changes, one which liberates energy or heat and thus tends to raise the temperature of the medium in which it occurs. See also Endothermic.



The product of extrusion; any cylindrical, prismatic, or pellet-shaped material so made. A useful form of a solid catalyst or catalyst substrate, for example; also, elsewhere in ceramics.



The process of forcing a plastic material or mixture through the opening(s) of a die at relatively high pressure. The material may thus be compacted (as in ceramic forming), and emerges in elongate cylindrical or ribbon (or wire, etc.) form having the cross section of the die opening. Ordinarily followed by drying, curing, activating, or firing.




Stepwise or gradual reduction in strength of a material when subjected to repeated or cyclic mechanical or thermal stresses somewhat below that causing fracture. Results from gradual growth of flaws.


FCC (Fluid used to Cracking Catalyst)

A bead form of catalyst accelerate cracking of hydrocarbons in refinery and similar operations, in which the catalyst is maintained as a fluid bed instead of stationary.



Generally a fine particulate solid, admixed in significant quantities into a product to provide bulk, color, opacity, dimensional stability, or other desired properties, or simply as an extender. See also Additive.



A porous membrane, sheet, plate, or bed of particles, with openings sized for a particular duty, used to effect the mechanical separation of a suspended solid from a gas, liquid, or slurry. Of many types: gravity, pressure or vacuum; stationary (e.g., plate-and-frame, bed), or moving as in rotary, belt, etc.


Filter Cake

The still-wet material collected on a filter, the excess liquid having passed through. That liquid is the "filtrate."


See Surface Finish.


Fire Retardant, Flame Retardant

A chemical diluent, filler, coating, or treatment used to retard or start or initial spread of flame inhibit the in a combustible material. Examples: alumina trihydrate, sodium borate, monium phosphate, numerous chloro and bromo organics.



Chemist's quantitative representation of a substance (compound or free element). See also Molecule, Repeat Unit. See Tables III and IV.


Fused Alumina

Alpha alumina made ordinarily by arcmelting and refreezing; crushed and sized, for example, in abrasive and refractory aggregate manufacture.



The process of melting; the transition or conversion from a crystalline solid state to the liquid state. "Freezing" is precisely the reverse.



Gamma Alumina

A particular crystalline transition alumina, derived thermally from boehmite and hence a major component of lightly calcined gel alumina. See also Substrate Alumina.



A colloidal state comprised of interdispersed solid and liquid, in which the solid particles or ligaments are themselves interconnected or interlaced in three dimensions. See also Sols.


Gel Alumina

Any of a family of porous, high-surfacearea aluminas and modified aluminas composed substantially of pseudoboehmite or boehmite, or derived from them by further processing; available in various convenient forms. See also Substrate Alumina.



A particular crystalline form of alumina trihydrate; occurs in bauxite but is most notable as the primary product of the Bayer process, hence the synthetic raw material from which aluminum and most aluminas of commerce are in turn derived. See Bayer Hydrate; also Reduction-Grade Ore.


Glass, Glassy

A state of matter that is amorphous or disordered like a liquid in structure, hence capable of continuous composition variation and lacking a true melting point, but softening gradually with increasing temperature. Glasses of commerce are mainly complex silicates in chemical combination with numerous other oxidic substances; made by melting the sourde materials together, forming in various ways while fluid, and allowing to cool. See Vitreous; also Polymer.



In ceramics, a mineral or chemical mixture usually applied as a fine suspension to the surface of a body, then dried and fired to produce a bonded, vitreous coating. Used for color, decoration, or sealing, See also Enamel, Glass.


As a subdivision of uncompacted solid materials, i.e., "granular," an irregular piece ranging in diameter from perhaps a centimeter or two to considerably less than a millimeter, with both limits depending on the usage. In refractories, refers to each individual piece or to a quantity of either loose or consolidated aggregate. In ceramic and metallic microstructures, grain is a single crystallite of the multitude comprising a polycrystalline material, these either self-bonded or bonded by some intergranular phase.


Grain Growth

In polycrystalline materials, a diffusioncontrolled, thermally activated phenomenon in which the larger grains grow still larger while the smallest ones usually diminish or disappear. See Recrystallization.


Grain Size, Grain-Size Distribution

Measures of the characteristic grain or crystallite dimensions (usually, diameters) in a compact polycrystalline solid; or of their populations by size increments from minimum to maximum. Usually determined by microscopy. See also Particle-Size Distribution.



The relative ease of reducing the particle size of a solid to some desired end-point by grinding; often stated as the amount of energy required, or the time required in a particular piece of equipment.



As opposed to crushing, the size reduction or comminution of solids into relatively small particles by mechanical means; generally, to diameters of the order of 1-2 mm and less. See Mill, Milling. Alternatively, a means of material removal used in shaping or finishing parts: see Abrasion, Abrasive.


Grinding Aid

An additive to a grinding system which promotes more efficient size reduction, usually by subtle alteration of the surface properties of the material being ground.


Grinding Media

Hard objects tumbled or propelled in a grinder or mill to conduct grinding action, such as balls, rods, or grains. See, e.g., Ball Mill.



Hammer Mill

A machine for grinding or pulverizing solids, in which free-swinging hammers are rotated at high speed within a stationary cylindrical grating. See Milling, Mill.


Hardness Resistance

of a solid material to indenting, also to abrasion; a measure of the cohesive forces between its atoms. See Mohs, Rockwell, DPH, Knoop, for measurement methods.



Of a body of material or matter, comprised of more than one phase (solid, liquid, and gas) separated by boundaries; similarly of a solid, comprised of more than one chemical, crystalline, and/or glassy species, separated by boundaries. See Homogeneous; also Composite.



Of a body of material or matter, throughout; hence, comprised of only one chemical composition and phase, without internal boundaries. (Homogeneous polycrystalline materials contain orientational boundaries only.)



A crystal or chemical substance containing chemically combined water, in (nearly) fixed proportions relative to its other constituents.

Hydrated Alumina

See Alumina Hydrate.



The act or process of taking up water chemically, as opposed to adsorption or absorption. Forming a hydrate.


Hydraulic, Hydrostatic

Force, pressure, or motion delivered by or through a liquid. Hydraulic cement is one made workable, and cured, by the action of water.



A kind of acid-base reaction with water which combines only its hydrogen (H⁺) or hydroxide ion (OH⁻), releasing the other one (hence changing the pH); infrequentlv. both at once.



In petroleum refining and similar organic chemical operations, the process of treating with hydrogen gas at high temperatures and pressures to effect desired chemical changes; usually employing a catalyst. Examples: hydrodesulfurization (HDS), hydrodenitrification (HDN), hydrocracking. See also Cracking.


The ability or tendency to take up moisture, usually from the air but hence from other moist materials as well. See, e.g., Desiccant.



Impurity, Impurities

In a chemical or material, minor constituent(s) or component(s) not included deliberately; usually to some degree or above some level, undesirable. See also Contaminant. 


Index of Refraction

See Refractive Index. 



Unreactive, stable, or indifferent to the presence of other materials; a relative term, usually applying under some limited set of circumstances or application. See also Reaction, Reactivity. 



Of chemical substances or mixtures, comprised other than of most compounds of carbon. See Organic, including limited exceptions. 


Insoluble, Insolubles

Not appreciably dissolved in water or some other specified reagent or solvent (e.g., acid, caustic, organics, etc.). A relative term of impurities, usually determined by a fixed procedure: see Acid Insols, Caustic Insols. See also Leach. 

Insulating Refractories

Highly porous (including fibrous) refractories used to efficiently contain or retard the passage of heat; often, behind or outside the working refractory which contacts the heated material.


Interfacial Tension

A measure of the adhesive forces between two phases (gas-solid, gas-liquid, liquid-solid, liquid-liquid) in contact. Usually determined by measuring the energy or work required to extend the area of contact by a unit amount. See also Surface Tension, Contact Angle.



A type of ceramic or powder-metallurgical forming or compaction process in which the mold is flexible and pressure is applied hydrostatically or pneumatically from all sides. 



Having the same value of a given property measured in any direction, or (sometimes) in all directions perpendicular to each other.




A fine, usually white clay material containing principally kaolinite, Al,Si,Os(OH); used in ceramics and refractories. Related minerals contained include nacrite and dickite, of the same formula.

Knoop Hardness

A "micro" hardness scale and instrumental measurement method: the depth of indent made by a rhombohedral diamond point of specified dimensions, when pressed into a surface under specified load. Used especially for materials harder than quartz.




The regular, repeated geometrical array of atoms, ions, or molecules (chemically bonded clusters of atoms) in a crystal. See Repeat Unit.



Calcium oxide, CaO; a refractory but moderately water-soluble and caustic substance usually obtained by calcining limestone, seashells, coral, or other forms of CaCO₃. Also called quicklime, burnt lime. Dolime contains about equimolar CaO, MgO. As-is or slaked (hydrated to Ca (OH)₂), lime has many used in materials, refractories, cements, chemicals, and chemical treatments.


A mineral rock or deposit consisting chiefly of calcium carbonate, CaCO₃. "Dolomitic limestone" contains some MgCO₃. See also Dolomite.


Loss on Ignition (LOI)

The fractional or percentage weight loss of a material on heating in air from an initial defined state (usually, dried) to a specified temperature such as 1000°C, and holding there for a specified period such as 1 hour. Fixed procedures are designed, usually, such that LOI represents the loss of combined H₂O, CO₂, certain other volatile inorganics, and combustible organic matter.



Macropore Volume

The volume fraction of a porous solid comprised of all connected pores or openings larger than a designated equivalent size, determined arbitrarily. In some alumina products, this limit is set at a diameter of 350 angstroms; in some, at 700 Å.



Magnesium oxide, MgO: calcined, or hardburned as periclase. Loosely, applied also to the hydrate, Mg(OH)₂. Made synthetically from seawater or brines, also (impure) from magnesite. Used in basic refractories, as a filler in rubbers, and elsewhere.



A mineral consisting chiefly of magnesium carbonate, MgCO₃; a source of "magnesite" as opposed to "periclase" basic refractories. The latter, made from synthetic MgO, are usually somewhat lower in silica and/or iron (typical contaminants of magnesite ores). See Magnesia; also Dolomite, Dolime.



In a polycrystalline and especially heterogeneous mass, e.g., igneous rocks and in refractories, the material between large crystals or grains of the major component(s), which bonds them together. May itself be crystalline or vitreous.


Mechanical Properties

Those characteristics of a material that relate force, load, or stress to deformation, flow, and/or fracture; e.g., elastic modulus, brittleness, ductility, malleability, yield stress, "toughness," creep, viscosity, various expressions of "strength," etc. 



To pass from a solid or rigid state to the liquid state; to fuse. Also, a quantity or batch or mass of liquid (sometimes also called a *bath"') contained in a vessel or otherwise localized.


Melting Point

The temperature (or very narrow range) at which a specified crystalline solid converts to a liquid and the reverse, or at which a specified solid and its liquid are at equilibrium. See Fusion Point, Freezing Point.


Mesh, Mesh Size

The opening(s) or size of opening(s) in a designated sieve or screen, hence the approximate diameter of particles below which they will pass through and above which they will be retained on the screen. "Mesh sizes" are given as number of wires per inch of standard screen construction, e.g., Tyler or U.S.; these are translated by tables into equivalent particle diameters in inches, millimeters (mm), or micrometers (µm or µ). See Table II.

Micrometer, Micron

(µm, µ) One-thousandth of a millimeter or one-millionth (i.e., 10⁻⁶) meter. See Table I.

Micropore Volume

The volume fraction of a porous solid comprised of all connected pores or openings smaller than a designated equivalent size, determined arbitrarily. In some alumina products, this limit is set at a diameter of 120 angstroms. See also Knudsen Diffusion. 



An English measure of thickness or diameter: 0.001 inch. A common designation of wire size, also of thin metal or ceramic sheets. 



A grinding machine or grinder, used for size reduction (comminution) of solids below "crushed" sizes, though overlapping. Of various mechanical types: ball or pebble mills, rod mills, roll mills, hammer mills, plate or disk mills, jet mills, etc. See Colloid Mill; also Crusher. 



The act or process of grinding to reduce the size of particulates, usually finer than by crushing. Often conducted dry, sometimes wet. See Mill.



A processing additive that promotes either the recrystallization or the partial fusion or sintering of certain mineral or ceramic materials, often thus facilitating the desired conversion at a lower temperature.



A formulated batch or ratio of different materials, usually sized, prepared as feed to a refractory, ceramic, glass, or other forming or manufacturing process, or fed into a reaction vessel. May be dry, wet, or even a slurry or suspension.


Mohs Hardness

A hardness scale originally developed for minerals; ranging from 1, that of talc, to 10, that of diamond, the hardest known substance. Determined by the ability of one substance to scratch another. The modified Mohs scale: 

1. Talc 

2. Gypsum

3. Calcite 

4. Fluorite

5. Apatite

6. Orthoclase feldspar

7. Fused silica

8. Quartz

9a. Garnet 

9b. Topaz

9c. Fused zirconia

9d. Fused alumina, corundum

9e. Silicon carbide

9f. Boron carbide

10. Diamond 

In the original scale, quartz was 7, garnet 8, corundum 9, and diamond 10, the others above 6 omitted.


MSA (Mine Safety Appliances)

A particular instrumental method of determining the subsieve particle-size distribution of a solid by sedimentation or settling in a liquid medium, using centrifuges at different speeds for specified durations.




One billionth (i.e., 10⁻⁹) meter. As an S.I. unit, preferred over the angstrom (10⁻¹⁰ m) for the representation of atomic and molecular dimensions and of the wavelengths of "'hard" electromagnetic radiation (e.g., X rays). "'Optical" wavelengths (uv, visible, and ir) may equally be represented in micrometers.

Neutral Refractories

Refractories that are about equally resistant to chemical attack by acidic or basic slags, melts, or fluxes. Alumina, alumina-chrome, spinel, zirconia, carbon, graphite, and silicon carbide are examples of near-neutral materials.




As properties or characteristics, having to do with radiation or the interactions of matter with radiation; especially of visible wavelengths but not necessarily so restricted.


Of compounds or mixtures thereof, the class comprised of most of the compounds of carbon. The exceptions are generally taken to include the carbides of metals and metalloids; certain solid interlayer or "edge" compounds of carbon or graphite; and CO, CO₂, and the derivative carbonates and bicarbonates (e.g., MyCO₃, MHCO₃). In fact, CO, CO₂, COS, and CS₂ may be regarded as organic or inorganic, depending on the chemical context. Similarly, elemental carbon may sometimes be regarded as organic in respect to its origins, but inorganic as a structural material.




In solid materials, a convenient, identifiable unit of subdivision present; ordinarily applied to loose or unconsolidated materials or to suspensions. May be a crystallite or crystal, a ground or milled fragment, a grain or other small agglomerate, etc.


Particle Density

See Density.


Particle Size

The size or size range (usually, equivalent diameter) of a given or typical particle, expressed either as nanometers, micrometers, millimeters, etc., or as mesh or sieve size. The ultimate particle size, the finest ordinarily achievable by grinding a polycrystalline material, sometimes equates to the size of its crystallites.


Particle-Size Distribution

The population of particles of a loose or suspended material for each size increment between the minimum and maximum size present; expressed either by increment or cumulatively, usually by weight fraction or weight percent. Especially important to the bulk density, and to the density achievable by compaction.


Pebble Mill

See Ball Mill.


Physical Properties

Those characteristics of any substance, material, or mixture which describe its nonchemical interactions with gravity (e.g., weight, density), heat, radiation (e.g., color, transmission, refraction), magnetism, electricity or electric fields, changes of state (e.g., melting, boiling), etc.; and often by inclusion, the mechanical properties. Thus, all properties except chemical and nuclear properties.



A subtype of adsorption in which the bonding of sorbate to the surface is weak (*physical") and the sorbate is not chemically altered; hence, easily reversed. See Chemisorption.


Plastic, Plasticity

Of a solid or particulate solid mix, the quality of readily deforming or flowing under applied force or pressure while remaining rigid under its own weight; hence ductile or capable of extrusion, ramming, tamping, or even trowelling. Alternatively, a loose term for an organic polymer material. See also Creep.


A processing additive which, by lubricity, imparts plasticity or workability to a material or mix that is otherwise difficult to deform.



A minute flat particle such as a tabular alumina crystal or typical clay particle. See Morphology.



Employing forced or compressed air or gas to transmit force, pressure, or motion.



To make smooth, often lustrous, by a mechanical process such as rubbing or friction, often using the finest abrasives. See Surface Finish.


Comprised of many crystals or crystallites, intimately bonded together. May be homogeneous (one substance) or heterogeneous (two or more different crystal types or compositions).



In crystals, one of two or more geometrical forms of the same composition, i.e., its atoms occupying one of two or more different lattices.


A hard, fine-grained, high-fired, vitreous (hence often translucent) ceramic body; used as table and ornamental wares, industrial whitewares (e.g., insulators), and chemical wares.


A small opening, void, interstice, or also channel within a consolidated solid mass or agglomerate; usually larger than atomic or molecular dimensions. See also Porosity.


Pore Size

The characteristic or equivalent dimensions or range of same (e.g., diameter) of a pore or family of pores in a material; usually measured indirectly by differential liquid (e.g., Hg) intrusion or cryogenic gas adsorption-desorption; or, microscopically on a section.


Pore-Size Distribution

The specific pore volume of all measurable pores in each of a set of equivalent diameter increments from the smallest to the largest present in a material; expressed either by increment or cumulatively. Most commonly measured by mercury intrusion at successively higher pressures. See Connected Porosity, Porosity.


Pore Volume, Specific Pore Volume

The porosity of a solid, otherwise subclassified, when expressed as cc/g or cm³/g or similarly.


Pore Volume Fraction

The porosity of a solid, otherwise subclassified, when expressed as cm³/cm³ or correspondingly in percent by volume.



Has meaning only for a consolidated form of solid, whether that be a particle, agglomerate, grain, formed object such as nodule, pellet, or larger monolithic mass or object. Any such single unit has a nominal volume, sometimes also called the envelope volume. Given that characteristic volume for reference, the porosity is the volume (hence, volume fraction) of pores contained in the solid within it. Since pores can be described in various specific ways, there is an equal number of corresponding expressions of porosity: 

  • Macroporosity See Macropore Volume. 

  • Microporosity See Micropore Volume. 

  • By Size Increment See Pore Size, Pore-Size Distribution. 

  • Connected or Open Porosity Also called Apparent Porosity. 

  • Closed or Blind Porosity.

  • Total Porosity The sum of Connected plus Closed Porosity. 

  • Porosity may also be expressed as determined by a given instrument or techniques, as, e.g.: 

  • Mercury Porosity which approximates Connected Porosity. 

  • Water Absorption which also approximates Connected Porosity. 

To eliminate the problem of determining nominal volume and also for engineering convenience, porosity with any of the above modifiers is often determined and expressed in cubic centimeters per gram (cc/g or cm³/g) instead of volume fraction. Porosity should not be confused with the interstitial volume or packing space between particles of an unconsolidated solid.



An industrial vessel, reactor, or apparatus in which a chemical or physical precipitation process (formation of a solid phase) is carried out in a liquid medium. Less commonly, a solid-gas separator.



An extremely fine-crystalline (perhaps 1 to 5 nm) boehmite, AlO(OH), precipitated as an indefinitely hydrous agglomerate by aqueous acid-base neutralizations involving aluminum or aluminate salts. Variable deliberately by choice of reagents and reaction and aging conditions. See Gel Alumina, Substrate Alumina.



Not a mill as ordinarily defined, but a mechanical mixing device consisting of one or more rotating shafts bearing arms or blades, within a drum or trough.



An instrument comprised of a vessel of calibrated volume, such that it can be filled (and weighed) with a given liquid alone, then with the same liquid plus a weighed quantity of a denser solid material (and weighed again). The displaced volume of liquid is calculated; this is the nominal or envelope volume of the solid if the liquid does not penetrate its connected porosity, or its apparent volume if the liquid does fill all connected pores.




A process, usually physical, by which one crystal species is grown at the expense of another or at the expense of others of the same substance but smaller in size. See Crystallization, Grain Growth; also Mineralizer.


Reduction-Grade Ore (RGO) or Alumina

A particular grade of calcined alumina developed especially for most effective use as feed in the manufacture of aluminum metal by the Hall electrolysis process.


Refractive Index

Refraction is the phenomenon of directional change of light (or Other electromagnetic) rays on passing from one medium into another. The refractive index of every liquid or solid substance or other homogeneous medium is a measure of this phenomenon when the second medium is air or vacuum; it is the inverse velocity of light (etc.) in that medium relative to that in air. It is function of wavelength, often of direction (see Birefringence, Anistropy) and of temperature.

Refractory, Refractories

"High-temperature-resistant"; hence, a large family of industrial lining and construction materials that are high melting and corrosionerosion-resistant when used as linings or walls of furnaces, kilns, reactors, and other vessels for the containment or processing of other materials (most often molten) at high temperature.


Relative Humidity

The water-vapor content per volume of air or other gas, divided by the saturation level at the same temperature and pressure, i.e., by the content that would be in equilibrium with liquid water. See also Dew Point.



Sanitary Ware

Porcelain enameled metal or else ceramic ware such as sinks, lavatories, and bathtubs. Impermeable at least at the surface.


Scale, Scaling

The formation of a layer of agglomerated or sintered solid material on the working surfaces of a reaction vessel, container, precipitator, drier, calciner, kiln, etc., whereas the processed material is intended to remain fluid or suspended. The layer of material so formed.


Scanning Electron Microscope (SE)

A high-power magnifying and imaging instrument using an accelerated electron beam as an optical device, and containing circuitry which causes the beam to traverse or scan an area of sample in the same manner as does an oscilloscope or TV tube. May utilize reflected (SEM) or transmitted (TEM) electron optics.



A wire mesh with specific sized openings for grading or separating various sizes of particulate or granular solids, or to "scalp" large agglomerates, rocks, and coarse tramp material. See Mesh, also Sieve.



An instrument for determining the particle-size distribution of a particulate solid, making use of a physical relation between rate of settling (' sedimentation') in a liquid and the particle size.



Small particles or agglomerates, crystals, or crystallites introduced in large numbers into a vessel to serve as nuclei or centers for further growth of material on their surfaces. See Crystallization, Agglomeration, Nucleation. Alternately, in glass, a small bubble (manufacturing defect); see also Cord, Stone.



The fractional reduction in dimensions or volume of a material or object when subjected to drying, calcining, or firing (sintering).


S.I. (Systeme Internationale) Units

The recommended set of "metric" units of measurement, now widely promulgated in tables and handbooks. Those in commonest use are shown here in Table I: the meter (m), gram or kilogram (g or kg), second (s), newton (N = kg * m/s²), pascal (Pa =N/m²), joule (J=N * m), watt (W= J/s), and temperature in kelvins (K) or in degrees Celsius (°C). See also decimal fractions and multiples given in the table.



A standard wire mesh or screen, especially when used in graded sets to determine the mesh size or particle-size distribution of particulate and granular solids. See also Subsieve. See Table I.



To density, crystallize, bond together, stabilize a particulate material, agglomerate, or prodby heating or firing close to but below the melting point. Often involves melting of minor components or constitutents, and/or chemical reaction. Also, the product of such firing. See also Clinker.



A slurry or suspension of fine clay or other ceramic powders in water, having the consistency of cream; used in slip casting or as a cement or glaze preparation.


Slip Casting

The ceramic forming process consisting of filling or coating a porous mold with a slip, allowing to dry, and removing for subsequent firing.



Any pourable suspension of a high content of insoluble particulate solids in a liquid medium, most often water.



Sodium carbonate (Na₂CO₃) or bicarbonate (NaHCO₃) or hydroxide (caustic soda, NaOH); or, any contained or combined form of sodium in a material, including as impurity, expressed as the equivalent Na₂O. See also Leachable Soda.


Sol-Gel Process

An important ceramic process operation in which a sol is converted to a gel by partial evaporation of the liquid phase and/or by neutralizing the electric charges on particles which cause them to repel each other. The gel is usually further processed (e.g., formed, dried, fired).



A homogeneous or single-phase, variable composition mixture of one substance (solute) in another (solvent), in which the former is dispersed as separated molecules, ions, or atoms. The solvent or the solution may be solid, liquid, or gas.



In general, the taking up of some substance (sorbate) into or on the surface of another (sorbent), without specification of the type of process. See Absorption, Adsorption, Chemisorption, Physisorption.



Prefix to many properties, as specific heat = heat capacity, specific resistance = resistivity, specific conductance = conductivity; specific volume, specific surface, etc. Refers to the given property expressed per unit weight or per unit volume of a material and/or otherwise for some reference conditions. Used for concise numerical tabulations of properties or coefficients, such that dimensions, etc., can be reintroduced in engineering equations derive statements of behavior in many and varied real situations.

Specific Gravity (sp. g.)

The ratio of the density of any material to the density of pure water at 4°C, i.e., the maximum density of water; infrequently, to the density of water at the same temperature as the material. Both densities must be expressed in the same units, hence the sp. g. is dimensionless. Subject to all the modifiers of the term "density" in use for solid materials, and having the same meaning, hence: Apparent Specific Gravity is obtained from the Apparent Density Bulk Specific Gravity is obtained from the Bulk Density Particle or Grain-Specific Gravity is obtained from the Particle Density Theoretical Specific Gravity is obtained from the Theoretical Density.



Any of a group of compounds of the same crystal type and general formula as magnesium aluminate, MgAl₂O₄ or MgO • Al₂O₃. That compound itself, which is refractory and chemically near-neutral. See Spinel Refractories.


Spinel Refractories

Refractory materials containing an aggregate of spinel; or, alternatively, "spinel-bonded" refractories comprised of periclase or alumina aggregate with a spinel bond or matrix.


Spray Dryer

A large vessel into which a slurry is sprayed through orifices in a stationary or revolving head and thrown as droplets into a stream of heated air which dries them. The dried droplets are typically tiny agglomerates, often in hollow bead form, hence freeflowing.


Standard, Standardized

Of a reagent or material, or an analytical or instrumental measurement meted or procedure, one which is fixed, specified, or established, often calibrated, so as to produce reliable results that are sensibly the same wherever used or employed.


Standard Practices Method(s), Manual

A set of standard or standardized analytical and test methods and procedures, adopted and catalogued by a given laboratory.



Restoring force of a body (or local part of it) when strained, i.e., deformed. Expressed in units of force/area, e.g., pascals.


Stress Corrosion

Corrosion at the tip of a flaw or crack, enhanced by a local state of stress, resulting in precritical crack growth or fracture.



Size In particulate solids, those sizes passing through a 325 mesh or 400 mesh screen or sieve and hence of diameters less than 44 or 37 µm. Sizes in this fine range are measured, classified, or analyzed by sedimentation methods and importantly by various types of "particle counters" based on other properties of suspensions. See Particle-Size Distribution; also Sieve Analysis.



A chemical entity (free element or compound, and crystal or other form of same) describable by a fixed formula giving the composition by relative numbers of atoms. Opposed to solution, mixture, etc., which are not of narrowly fixed composition. See Use of Tables of Chemical Elements; also Valence.



A body, board, or layer of material, on which some other active or useful material(s) or component(s) may be deposited or laid, as, for example, in electronic circuitry laid on an alumina ceramic board. In catalysts, the formed, porous, high-surface-area carrier on which the catalytic agent is widely and thinly distributed for reasons of performance and economy.


Substrate Alumina

Alumina especially for use as substrate in the second definition given above; and, particularly, such an alumina originating as gel alumina, hence as pseudoboehmite or boehmite. After forming, catalyst application, and activating or calcining, likely to be gamma or other transition alumina. Other useful substrates of alumina may be made from active alumina or still other forms. See also Nodules and Extrudates as to geometrical shapes.



Of solids, the external boundary; or, alternatively, the total of external and all internal surfaces bounding pores, voids, channels, crevices, cracks, etc.


Surface Activity, Acidity, Basicity

The extent and degree of adsorptivity or reactivity (acidity, basicity, etc.) of a given area of accessible solid surface, resulting from chemical modification and crystalline or other imperfections thereon. Aluminas are capable of extensive tailoring of these properties. See also Acid Site, Basic Site.


Surface Area

The area, per unit weight of a granular or powdered or formed porous solid, of all external plus internal surfaces that are accessible to a penetrating gas or liquid. See Connected Porosity, also BET.


Surface Finish

Degree of roughness or of surface damage left by a finishing operation (grinding, polishing, etc.). Apart from obvious importances, may control the severity of surface flaws.


Surface Tension

A measure of the cohesive forces among surface molecules, etc. Generally represented by energy per unit surface area, sometimes by force per unit edge length. See Interfacial Tension.


Synergism, Synergistic

A quality of mutual reinforcement of some property by two components or agents, such that the property of some mixture of the two is more favorable than the linear combination or weighted average between them. Also can be extended to three or more components. Often exhibited, for example, by aluminas in combination with additives, modifiers, or promoters.




From "table": platelike or platelet in crystal morphology or habit.


Tabular Alumina

High-fired, dense alpha alumina, whose relatively large crystals or grains are predominantly of tabular or platelet morphology.


Theoretical Density, Specific Gravity

See Density, Specific Gravity.


Theoretical Volume

Of a quantity of a solid material, the volume it would occupy if all voids were removed and the material were at 100% of its true or theoretical density. See Diffraction as a means of measuring.


Thermal Conductivity

A measure of the ability to transmit heat through a mass of given material, expressed as heat flux per unit of temperature gradient, either at a given base temperature or by a temperaturedependent equation.


Thermal Expansion

(Coefficient of) The linear or volume expansion of a given material per degree rise in temperature, expressed at an arbitrary base temperature or as a more complicated equation applicable to a wide range of temperatures.


Thermal Shock

A large and rapid temperature change, resulting in large temperature differences within or across a body. See Thermal Stress.

Thermal Stress

Internal stresses developed within a body by virtue of temperature gradients or differences and accompanying thermal expansion or contraction effects. Can result in fatigue, fracture, or spalling in brittle materials.


Thermogravimetric Analysis (TGA)

An instrumental method of continuously recording the weight of a material sample (exposed to controlled atmosphere or vacuum) while its temperature is gradually raised and recorded. Gives evidence of evaporation and of reactions (e.g., combustion, decomposition) producing volatile products. See also Differential Thermal Analysis.


Thixotropic, Thixotropy

Of colloidal suspensions, the property of decreasing in viscosity (i.e., of becoming more fluid) under increasing shearing force or speed. See also Dilatant.



A physical change from one crystalline form to another or a change of state (solid, liquid, gas), in any direction.


Transition Aluminas

A family (several series) of metastable crystalline alumina containing minor but detectable OH⁻ as well as O₂-ions. Produced more or less sequentially by gradual heating of the different alumina trihydrates and monohydrates; also in some cases, synthesized by other processes. Examples are active alumina, substrate alumina, and the light-burned calcines.



Transmitting some incident light, but not clear images due to scattering within the material.


Tunnel Kiln

A direct-fired (less often, indirect-heated) continuous kiln in the form of a stationary tube or tunnel, through which cars containing ceramic or refractory wares are pushed as a train. Usually "zoned" for controlled heating and cooling rates and for soaking at the maximum temperature.




A measure of the resistance of a fluid (liquid or gas) or semifluid material to motion in shear, viz., one layer across another; determined by specified instrumental methods.



Free Water

That fraction or percentage composed of H₂O that is present as "moisture," "dampness," or "wetness": i.e., absorbed and occluded.


Bound Water

Especially in high-surface-area materials, that fraction or percentage composed of H,O that is adsorbed (subdivided as physisorbed or chemisorbed) on the surface. Chemisorbed and combined water as defined here may, however, be quite difficult to distinguish; and indeed the distinction between adsorbed and absorbed water may not be sharp either. Hence, often, a resort to arbitrary analytical methods.

Water Absorption

The amount of liquid water absorbed by a consolidated (though porous) solid under a fixed test procedure; commonly expressed as weight percent of the dry material. Can be converted arithmetically to cm³/g or to vol% as a measure of porosity.



X ray

Radiant energy falling within a band of wavelengths or frequencies, or energy quanta, corresponding to any of numerous electronic transitions within excited atoms. Shorter in wavelengths and more penetrating than uv; ionizes matter. Useful in numerous instrumental and analytical methods including diffraction and various kinds of spectrometry.




Zirconium oxide, ZrO₂, in any of several crystal forms. Used as a structural ceramic, "oxygen sensor" in analyzing combustion gases, refractory material, catalyst, or substrate, ceramic modifier, etc.

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