Fluorspar was formerly used to some extent in the manufacture of plate, bottle, and window glass to assist in melting the batch and to make it more fluid. It is little used to-day for this purpose and opinions differ as to its value.

Fluorspar when used in glass manufacture is usually ground so that it will pass a 100-mesh screen, but some is much finer and some as coarse as 40 mesh. It is usually packed in bags or barrels, but is sometimes shipped in bulk in tight box cars lined with building paper. At glass factories it is stored in a dry place, either in packages or in bulk in wooden or concrete bins or silos conveniently located near the mixing room.

Opal-glass batches vary greatly and may contain 50 to 500 pounds of fluorspar per 1,000 pounds of sand. A typical batch for a rather dense opal glass, as given by one manufacturer, has about the following composition:

A manufacturer of opal glass in plate and slab form states that he uses 250 pounds of fluorspar per 1,000 pounds of sand. The fluorspar is used as a source of fluorine, which with alumina supplied by feldspar, kaolin, or lepidolite, imparts an opalescent whiteness to the glass. This color varies from light opalescence to a dense dead white, depending upon the amount of fluorine present.

GRADES AND IMPURITIES

There are no standard specifications for fluorspar for glass manufacture, and widely varying grades are in use. The most objectionable impurity is iron, which makes the glass greenish or yellowish brown. One large company sets a limit of 0.12 per cent of iron, expressed as the oxide (FeO3), and states that the average iron oxide content of the fluorspar it uses is about 0.06 per cent.

Silica is not an injurious impurity, as it is the chief ingredient of the batch (in the form of sand), but in excess forms a diluting agent in a rather expensive material. If proper price adjustments were made a fairly high silica content would probably be acceptable. One company reported using a Canadian fluorspar containing over 13 per cent silica. Another company stated that its low limit was 90 per cent CaF, and that it would object to receiving many cars averaging below 95 per cent CaF2. However, under present conditions a standard glass-grade spar should contain at least 96 per cent CaF, and not over 2 to 22 per cent SiO2. This grade is known as No. 1 ground or A-1 ground.

Calcium carbonate is an objectionable impurity in that excess of calcium oxide causes breakage of the glass or brittleness, or "hardness as the glassmaker calls it. Most glassmakers object to over 1 per cent CaCO, and if as much as 1 per cent is present it should not vary within wide limits. It is reported, however, that one western fluorspar producer shipped considerable quantities of fluorspar for opal-glass manufacture, containing about 2.5 per cent CaO3, 3.5 per cent SiO2, and 94 per cent CaF2.

Lead and sulphur are objectionable impurities in that they require additional expensive oxidizing materials. to neutralize or remove them. Barite and sphalerite would probably be objectionable on the same grounds. It is also probable that excessive quantities, say over 1 per cent, of any of these materials should cause rejection.

SUBSTITUTES

Fluorspar in glass manufacture is used as a source of fluorine, which may be supplied by other fluorides, such as cryolite (Na,AIE). Cryolite is used extensively in glass manufacture; in fact, fluorspar is usually considered a substitute for cryolite rather than the

reverse.

Fluorspar is probably used in glass manufacture more extensively than cryolite. Some glassmakers consider it much superior to cryolite, while others consider the use of too much fluorspar dangerous due to its usual content of calcium carbonate. Under present conditions, fluorspar is the cheapest source of fluorine.

Several manufactured fluorine salts, such as artificial cryolite, sodium silicofluoride (Na,SiF, containing 60.6 per cent fluorine), and sodium fluoride (NaF, containing 45.3 per cent fluorine), are also used for this purpose. The chief value of these artificial salts lies in their purity, which allows them to be used by nontechnical men without mathematical calculations to allow for variation in composition of natural minerals.

The mineral lepidolite, used by some glassmakers, contains fluorine possibly up to 5 per cent and thus allows the fluorspar content of a batch to be reduced. It is used chiefly, however, as a source of alumina, potash, and lithia.

Bone ash and other calcium phosphates are used to some extent in Europe and formerly in this country in place of the fluorides in opalescent glass. This method, however, is now considered more expensive and less satisfactory than the use of fluorides.

Most manufacturers of opalescent glass consider fluorspar an essential raw material.

USE IN ENAMELS

Fluorspar is an important constituent in dense, opaque, white, and colored enamels used in coating sheet iron and steel, and cast-iron sinks, tubs, trays, kitchenware, and so on. It is also used in making enameled brick and tile and for other ceramic purposes where an opaque, easily fusible enamel is needed. The type and grade of fluorspar, methods of shipping and storing, effect of impurities, and so on are the same for this use as for making opal glass.

Fluorspar alone or with cryolite is used in enamel batches in quantities ranging from 2 to 15 per cent of the batch. Probably 5 to 7 per cent represents a fair average. Most enamelers consider fluorspar an essential, but some believe that cryolite is an adequate substitute, while others do not consider cryolite a substitute for fluorspar. Other possible substitutes are artificial cryolite, sodium fluoride, barium fluoride, and sodium silicofluoride.

HYDROFLUORIC ACID AND FLUORINE CHEMICALS

Large quantities of fluorspar are used in the manufacture of hydrofluoric acid (HF). For this purpose fluorspar is sold either ground to 80 to 100 mesh or in lump or gravel form (acid lump, acid gravel, or "Keystone" grade) and is ground at the consumer's plant. The requirements for acid-grade fluorspar vary slightly but usually approximate the following specifications: CaF, not less than 972 to 9812 per cent, SiO, not over 1 to 12 per cent, and CaCO, not over 1 to 14 per cent. These specifications usually must be strictly observed.

The manufacture of hydrofluoric acid consists essentially of treating ground fluorspar with concentrated sulphuric acid. The reaction involved, which is

CaF2+H2SO4=2HF+CaSO4,

will take place at ordinary temperatures, but complete dissociation is effected only at or above 130° C. The reaction is carried on in platinum or cast-iron retorts and the hydrofluoric acid, which distills off as a vapor, is collected in water in lead-lined condensers. Commercial acid consists of an aqueous solution of HF. Commercial acid grades are 30, 40, 48, and 52 per cent, and fuming.

Recent descriptions of modern methods of hydrofluoric acid manufacture are not available. Betts 17 has described methods in use some years ago. Fickes 18 describes a continuous method. Hy

17 Betts, Anson G., "The manufacture of hydrofluoric acid": Eng. and Min. Jour., vol. 83, p. 753.

18 Fickes, Edwin S., Manufacturing of Hydrofluoric Acid: U. S. Patent 1316569, Sept. 23, 1919; Chem. and Met. Eng., vol. 22, No. 2, Jan. 14, 1920, p. 90.

drofluoric acid is used chiefly in etching glass and in the manufacture of fluorine chemicals.

Hydrofluosilicic acid (H2SiF) is usually made by heating pure silica sand with powdered fluorspar and sulphuric acid in excess, is distilled as a gas and collected in distilled water, and is used chiefly as a source of the fluorsilicates. Magnesium fluorsilicate or magnesium silicofluoride (MgSiF.) is made by treating magnesium hydroxide or carbonate with hydrofluosilicic acid. It is used to some extent in concrete hardeners.

Sodium fluoride (NaF) is made by treating sodium carbonate with hydrofluoric acid. It is used in ceramics, as a food preservative, as an antiseptic, as an antifermentative in alcohol distilleries and so on, and as a wood preservative.

Sodium silicofluoride, or sodium fluorsilicate (Na,SiF) is made by neutralizing hydrofluosilicic acid with sodium carbonate. It is used in ceramics and in medicine, and is reported to be used as a substitute for oxalic acid for certain bleaching purposes.

Calcium silicofluoride or fluorsilicate (CaSiF) is made by the action of hydrofluosilicic acid on calcium carbonate and by subsequent recrystallization. It is used chiefly in ceramics.

Barium fluoride (BaF2) is made by treating barium sulphide with hydrofluoric acid, followed by crystallization. It is used in enamels, as an antiseptic, and in embalming fluids.

Potassium fluoride, KF (anhydrous) or KF.2H2O (crystalline), is made by saturation of hydrofluoric acid with potassium carbonate. It is used in etching glass, and as a wood preservative.

OPTICAL FLUORSPAR

A relatively small but important use for clear, transparent, nearly colorless fluorspar is for the manufacture of certain types of lenses. Pogue 19 has written an excellent description of optical fluorspar:

Clear, colorless, or faintly colored specimens, such as occur rather sparingly in some localities along with the crude material, are suitable for the manufacture of certain types of lenses and prisms employed in microscopes and other optical instruments. Although the largest known deposits of fluorite in the world occupy a belt of country extending from southeastern Illinois into western Kentucky, but centering in Hardin County, Ill., the availability for optical use of the product from this region has heretofore been neglected, and the United States has been dependent upon foreign sources for its material, which came largely through the hands of the German optical dealers.

In connection with a recent geological survey of the fluorspar deposits of southern Illinois, the State geological survey, with this application in mind, has determined the presence of optical fluorite of excellent quality and in quantity probably sufficient to supply the needs of the United States.

19 Pogue, Joseph E., Optical fluorite in southern Illinois: Univ. of Illinois, State Geol. Surv. Div. Bull. 38, 1918, 6 pp.

Pogue has discussed the properties, uses, and value of optical fluorite in great detail.

OTHER USES

Large amounts of a rather low-grade acid fluorspar are employed in the manufacture of artificial cryolite, which is used with natural cryolite in the molten bath in which metallic aluminum is made by electrolysis. Artificial cryolite is made and used in this country only by the Aluminum Ore Co. (subsidiary of the Aluminum Co. of America). No details of the actual method of use are available. The artificial cryolite is reported to be less efficient than the natural mineral, due to an excess of water, so that some natural cryolite (possibly 20 to 25 per cent) must be used. Much artificial cryolite is made in Europe.

Massive fluorspar has been used to some extent as an ornamental stone, principally in Europe. Blue massive fluorspar found at Derbyshire, England, and known as Derbyshire spar or "blue John " is cut and shaped on lathes to make paper weights, vases, and other ornamental objects. Clear, colored fluorspar crystals are sometimes cut into gems for cheap jewelry, but they are too soft to have much value.

Minor uses of fluorspar reported are as follows: In the electrolytic refining of antimony and lead; as a flux in place of borax in melting gold and silver concentrates; as an ingredient of the bond for abrasive grains in manufacturing abrasive wheels; as a flux in the manufacture of alundum and other artificial abrasives and refractories made in electric furnaces; in the manufacture of carbon electrodes used for flaming-arc lamps and other purposes; in increasing the recovery of potash and in promoting fuel economy by reason of its fluxing action in the manufacture of Portland cement; and as a flux in the manufacture of white Portland cement.

Fluorspar "glass," 20 used for joining glass tubes of different melting points, such as quartz and pyrex glass to lead glass, is now made by a company in the United States. Several years ago a "glass" made from fluorspar, known as Fritsch's glass, was produced in Germany, but little practical application was found for it. This material had to be melted in platinum crucibles and cast in platinum molds, a process said to be very destructive to the platinum.

SODIUM FLUORIDE AS A WOOD PRESERVATIVE

Sodium fluoride is recognized as a very valuable wood preservative, but wider use has been restricted by high prices. For this purpose it has several minor advantages over zinc chloride, such as lower

20 Glass Industry, "Answer to question": Vol. 3, No. 10, Oct., 1922, p. 209.