GLAZING, the encrusting of earthen ware with a vitreous substance, the basis of which is lead.

The workers of common earthen ware, however, are not at the trouble of thus previously making a pure glass of lead. Their usual composition for glazing ware is formed, of white sand forty pounds, of red lead twenty pounds, of pearl ashes twenty pounds, and of common salt twelve pounds. The sand is powdered by grinding, and then the other ingredients are added and ground together; after which they are calcined for some time with a moderate heat, and when the mixture is cold it is reduced to powder, and when wanted for use tempered 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 in general, however, 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 forty pounds of white sand, twenty-five pounds of pearl ashes, and fifteen pounds of common salt, and proceeding as before; and a more perfect transparent glazing may be made of sand forty pounds, of wood ashes perfectly burned fifty pounds, of pearl ashes ten pounds, and of common salt twelve pounds. The receipts annexed are taken for the most part from Kunckel, who says that they are the true glazings used at Delft, and in other Dutch manufactories.

Black is made of red lead eight parts, iron filings three parts, copper ashes three parts, and zaffer two measures. This when melted makes a brown black; and if it be wanted blacker, more zaffer must be added 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 several times, quenching them always in cold water. If it be desired to have it fine and good, 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 must 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 zafser, 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 fourteen parts, and of manganese two parts, fused; or of red lead twelve parts, and manganese one part, fused. A brown glazing, to be laid on a white ground, mav be made of manganese two parts, and of red lead and white glass one part of each, twice fused. Flesh-coloured is made of twelve parts of lead ashes, and one of white glass. To prepare the gold-coloured, take of litharge three parts, of sand or calcined flint one part; pound and mix these well together, then run them into a yellow glass with a strong fire. Pound [7:10:583]this glass, and grind it into a subtile powder, which moisten with a well-saturated solution of silver; form it into a paste, then put it into a crucible, and cover it with a lid. Apply at first a gentle degree of fire; then increase, and continue it until a green glass has been obtained. Pound this glass again, and grind it to a fine powder; moisten the 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 which are painted or covered over with this glazing must first be well heated, then put under a muffle; and as soon as the glass runs, they must be smoked, by holding them over burning vegetables, and then taken out. Heinsius of St Petersburg, 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, sect. 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 twelve parts, and of filings of iron one part.

Green may be prepared from eight parts of litharge or red lead, eight parts of Venice glass, and four parts of brass dust or filings of copper; or from ten parts of litharge, twelve of flint or pebble, and one of as ustum or copper ashes. A fine green glazing may also be produced by fusing one part of the Bohemian granate, 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, and the same quantity of red lead, and of filings of copper, powdering the mass, and adding one part of Bohemian granate to two parts of this powder. A fine green may likewise 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 obtained 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 from fifteen parts of lead ashes or red lead, fifteen of white sand or flints, and five of calcined copper. This mixture must be calcined and fused. Liver colour is prepared from twelve parts of litharge, eight of salt, six of pebble or flint, and one of manganese. Purple brown consists of lead ashes fifteen parts, clean sand or powdered flints eighteen parts, manganese one part, and white glass fifteen 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, which are ground 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.

The white glazing for common ware is made of forty pounds of clear sand, seventy-five pounds of litharge or lead ashes, twenty-six of potashes, and ten pounds of salt; and these are thrice melted into a cake, quenching it each time in clear cold water. Jθr it may be made of fifty pounds of clean sand, seventy of lead ashes, thirty of wood ashes, and twelve 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 one part each, and fusing the mixture. A white glazing may also be prepared by mixing a hundred pounds of mastico, sixty pounds of red lead, twenty pounds of calcined tin or putty, and ten pounds of common salt, and calcining and powdering the mixture several times.

Yellow is prepared of red lead three pounds, calcined antimony and tin two pounds of each; or, according to some, of equal quantities of the three ingredients. These must be melted into a cake, then ground to a fine powder, and this operation must be repeated several times; or it may be made of fifteen parts of lead ore, three parts of litharge of silver, and fifteen parts of sand. A fine yellow glazing may be procured by mixing five parte of red lead, two parte 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 sixteen parts of flints, one part of iron filings, and twenty-four parte of litharge. A light-yellow glazing may be produced with ten parte of red lead, three parte of antimony and three of glass, and two parte of calcined tin. A citronyellow is made of six parts of red lead, seven parte 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.

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 dangerous impregnation from the metal. The Roman glazing, which is still to be seen upon urns excavated 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 was not subject to any of the accidents which may ensue from the use of vessels glazed with lead.