Talc is a hydrated silicate of magnesium having the theoretical formula H2Mg3 (SiO3)4, corresponding to 63.5 per cent SiO2, 31.7 per cent MgO. The ratios of these constituents may vary between rather wide limits, the magnesia/silica ratio ranging between 1.0 and 1.3. Ordinary talc is foliated but the laminae, although flexible, are not elastic. It may vary in colour from white to greenish-grey and dark green, and its specific gravity ranges between 2.6 and 2.8. The refractive index is usually between 1.54 and 1.59. It has a greasy feel to the touch and a hardness of 1 to 1.5. As talc usually occurs as a secondary mineral resulting from the alteration of other magnesium silicates or carbonates, such as serpentine, tremolite, pyroxene, or dolomite, commercial talc may contain a proportion of these minerals. In general, the objectionable impurities in commercial talc are unaltered amphibole, pyroxene, quartz, iron oxides, and pyrite. Most commercial talc is ground as mined and without previous beneficiation, in many cases only handpicked material being submitted for size reduction. In the trade, the name “Talc” is often used to designate: (a) The mineral in the form of an aggregate of flakes and fibres. (b) The compact crypto-crystalline variety known as steatite. (c) A rather soft, impure talcose rock of variable talc content termed soapstone. Concentration of talc is not extensively practised, most talcs being handpicked and ground either wet or dry. Talc flotation is sometimes employed for the production of specific talc products. Dry grinding of the talc appears the most common method of size reduction, the gritty materials such as dolomite being removed by air flotation or classification during the process. Wet grinding has the disadvantage of destroying the “slip” which is an important quality of most commercial talcs. The name “French Chalk” was at one time largely used to designate prepared talc, but its use is rapidly disappearing; however, the name of “agalite” still persists in the paper trade and refers to ground talc. The term “asbestine” is reserved for a fibrous talc mined in the United States, and used for specific purposes in paint manufacture. “Steatite” or “lava talc” is the name applied to compact massive talc which has no visible grain. It can be sewn, bored, drilled or machined to any required shape. The required physical properties of talcs to make them useful in a wide variety of industries are as follows: (a) Extreme softness and smoothness (b) Good lustre (c) Extreme whiteness (d) High degree of “slip” (e) High fusion point (f) High absorption of oil and grease (g) Low electrical and heat conductivity (h) High specific heat.