the crusting over earthen ware with a vitreous substance, the basis of which is lead. See Glass of Lead.

The workers of common earthen ware, however, are not not at the trouble of thus previously making a pure glass of lead. Their usual composition for glazing their ware is formed of white sand 40 pounds, of red lead 20 pounds, of pearl ashes 20 pounds, and of common salt 12 pounds. Powder the sand by grinding it, and then add it to the other ingredients and grind them together; after which calcine them for some time with a moderate heat, and when the mixture is cold, pound it to powder; and when wanted for use temper it with water. The proportion of these ingredients may be occasionally varied. The ware after being turned on the wheel and dried in the open air, is covered over with the above composition by means of a brush; and when set in the furnace the violent heat soon reduces it to a perfect glass, covering the whole internal and external surface of the vessel.

We may observe, however, in general, that lead ought to be excluded from the composition of glazings, and other fluxes substituted in its stead. A transparent glazing may be prepared without lead, by calcining 40 pounds of white sand, 25 pounds of pearl ashes, and 15 pounds of common salt; and proceeding as before; and a more perfect transparent glazing may be made of sand 40 pounds, of wood ashes perfectly burnt 50 pounds, of pearl ashes 10 pounds, and of common salt 12 pounds. The following receipts are taken for the most part from Kunckel, who says, that they are the true glazings used at Delft and other Dutch manufactories.

Black is made of eight parts of red lead, iron filings three, copper ashes three, and zaffer two measures. This when melted will make a brown black; and if you want it blacker, add more zaffer to it.

Blue is thus prepared: Take lead ashes or red lead one pound, clear sand or powdered flints two pounds, common salt two pounds, white calcined tartar one pound, Venice or other glass half a pound, zaffer half a pound; mix them well together and melt them for several times, quenching them always in cold water. If you would have it fine and good, it will be proper to put the mixture into a glass furnace for a day or two.

Another blue glazing may be formed of one pound of tartar, a quarter of a pound of red lead, half an ounce of zaffer, and a quarter of a pound of powdered flints, which are to be fused and managed as in the last receipt. Or, take two pounds of calcined lead and tin, add five pounds of common salt, five pounds of powdered flints, and of zaffer, tartar, and Venetian glass, each one pound. Calcine and fuse the mixture as before. Or, again, take of red lead one part, of sand three parts, and of zaffer one part. For a violet blue glazing, take four ounces of tartar, two ounces of red lead, five ounces of powdered flints, and half a drachm of manganese.

Brown is made of red lead and flints of each 14 parts, and of manganese two parts fused; or of red lead 12 parts, and manganese one part fused. A brown glazing, to be laid on a white ground, may be made of manganese two parts, and of red lead and white glass of each one part, twice fused.

Flesh coloured is made of 12 parts of lead ashes, and one of white glass.

Gold coloured. Take of litharge three parts, of sand or calcined flint one part; pound and mix these very well together, then run them into a yellow glass with a strong fire. Pound this glass, and grind it into a subtile powder, which moisten with a well saturated solution of silver; make it into a paste, which put into a crucible, and cover it with a cover. Give at first a gentle degree of fire; then increase it, and continue it till you have a glass, which will be green. Pound this glass again, and grind it to a fine powder; moisten this powder with some beer, so that by means of a hair pencil you may apply it upon the vessels or any piece of earthen ware. The vessels that are painted or covered over with this glazing must be first well heated, then put under a muffle; and as soon as the glass runs, you must smoke them, by holding them over burning vegetables, and take out the vessels. Mr Heinsius of Petersburgh, who sent this receipt to the Royal Society, uses the words afflare debes fumum, which is rendered smoke them, in the Transactions. Phil. Trans. No 465.

§ 6.

Kunckel gives several preparations for a gold coloured yellow glazing. This may be produced by fusing a mixture of three parts of red lead, two parts of antimony, and one part of saffron of Mars; by again melting the powdered mass, and repeating the operation four times, or by fusing four or five times a composition of red lead and antimony of each an ounce, and of scales of iron half an ounce: or by calcining and fusing together eight parts of red lead, six parts of flints, one part of yellow ochre, one part of antimony, and one part of white glass. A transparent gold coloured glazing may be obtained by twice fusing red lead and white flints, of each 12 parts, and of filings of iron one part.

Green may be prepared of eight parts of litharge or red lead, eight parts of Venice glass, four parts of brass dust or filings of copper; or of ten parts of litharge, twelve of flint or pebble, and one of asaustum or copper ashes.—A fine green glazing may be produced by fusing one part of the Bohemian granite, one part of filings of copper, one part of red lead, and one part of Venetian glass; or by fusing one part of white glass, the same quantity of red lead, and also of filings of copper; powdering the mass, and adding one part of Bohemian granite to two parts of this powder. A fine green may be obtained by mixing and grinding together any of the yellow glazings with equal quantities of the blue glazings; and all the shades and teints of green will be had by varying the proportion of the one to the other, and by the choice of the kind of yellow and blue.

Sea green is made of five pounds of lead ashes, one pound of tin ashes, three pounds of flint, three quarters of a pound of salt, half a pound of tartar, and half a pound of copper dust.

Iron colour is prepared of 15 parts of lead ashes or red lead, 15 of white sand or flints, and five of calcined copper. This mixture is to be calcined and fused.

Liver colour is prepared of 12 parts of litharge, eight of salt, six of pebble or flint, and one of manganese.

Purple brown consists of lead ashes 15 parts, clean sand or powdered flints 18 parts, manganese one part, and white glass 15 measures, to which some add one measure of zaffer. Red is made of antimony three pounds, litharge or red lead three, and rust of iron one; grind them to a fine powder. Or, take two pounds of antimony, three of red lead, and one of calcined saffron of Mars, and proceed as before.

White. The white glazing for common ware is made of 40 pounds of clear sand, 75 pounds of litharge or lead ashes, 25 of pot ashes, and ten pounds of salt: these are three times melted into a cake, quenching it each time in clear cold water. Or it may be made of 50 pounds of clean sand, 70 of lead ashes, 30 of wood ashes, and 12 of salt.

For a fine white: Take two pounds of lead and one of tin; calcine them to ashes; of this take two parts, calcined flint, white sand, or broken white glass, one part, and salt one part; mix them well together and melt them into a cake for use. The trouble of calcining the tin and lead may be prevented by procuring them in a proper state.

A very fine white glazing may be obtained by calcining two parts of lead and one part of tin; and taking one part of this mass, and of flints and common salt of each one part, and fusing the mixture.

A white glazing may be also prepared by mixing 100 pounds of masticor, 60 pounds of red lead, 20 pounds of calcined tin or putty, and 10 pounds of common salt, and calcining and powdering the mixture several times.

Yellow is prepared of red lead three pounds; calcined antimony and tin, of each two pounds; or, according to some, of equal quantities of the three ingredients. These must be melted into a cake, then ground fine; and this operation repeated several times; or it may be made of 15 parts of lead ore, three parts of litharge of silver, and 15 parts of sand.—A fine yellow glazing may be procured by mixing five parts of red lead, two parts of powdered brick, one part of sand, one part of the white glazings, and two parts of antimony, calcining the mixture and then fusing it. Or, take four parts of white glass, one part of antimony, three parts of red lead, and one part of iron scales, and fuse the mixture; or fuse 16 parts of flints, one part of iron filings, and 24 parts of litharge. A light yellow glazing may be produced with ten parts of red lead, three parts of antimony, and three of glass, and two parts of calcined tin. See Gold colour above.—A citron yellow is made of six parts of red lead, seven parts of fine red brick dust, and two parts of antimony. This mixture must be calcined day and night for the space of four days, in the ash hole of a glass-house furnace, and at last urged to fusion.

For the glazing of Delft ware, Porcelain, Stone-ware, &c., see the articles Delft Ware, Porcelain, and Pottery.

The Romans had a method of glazing their earthen vessels, which in many respects appears to have been superior to ours. The common brown glazing easily scales off, cracks, and in a short time becomes disagreeable to the eye. Besides, it is very easily destroyed by acids; nor can vessels glazed in this manner be even employed to hold water, without part of it oozing through their pores. Lead is also very destructive to the human body; and if acids are unwarily put into vessels glazed with lead, the liquors will receive a very dangerous impregnation from the metal. The Roman glazing, which is yet to be seen upon urns dug up in several places, appears to have been made of some kind of varnish; and Pliny gives us a hint that it was made of bitumen. He tells us that it never lost its beauty, and that at length it became customary to glaze over statues in this manner. As this varnish sunk deep into the substance of the ware, it was not subject to those cracks and flaws which disfigure our vessels; and as it was not liable to be corroded by acids, it could not be subject to any of the accidents which may ensue from the use of vessels glazed with lead.