GLAZING, the coating over earthen ware by a vitreous substance, the basis of which is lead.
For making a pure glass of lead, the following receipt will be found to answer: Put a large quantity of lead into a potter's kiln, and keep it in a state of fusion with a moderate fire, till it is calcined to a grey loose powder; then spread it in the kiln, and give it a greater heat, continually stirring it to keep it from running into lumps. Continue this for several hours, till the powder becomes of a fair yellow; then take it out and sift it fine. This is called calcined lead.—Take of this calcined lead, 15 pounds; and crystalline or other flint, 12 pounds: mix these as well as possible together; put them into a pot, and set them in a furnace for 10 hours; then cast the whole, which will now be perfectly melted, into water; separate the loose lead from it, and return the metal into it; and after standing in fusion 12 hours more, it will be fit for use.
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 their ware is formed of sand, wood-ashes, lead-ashes, and salt. The ware, after being turned on the wheel, and dried in the open air, is covered over with this 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.
The various colours for glazing are the same with those for tinging glass, which we have already described. A black colour is given with manganese or iron. The proportions, however, are not exactly ascertained, nor are there any receipts published concerning these things on which we can much depend. In the Philosophical Transactions we have the following receipt for a fine gold-coloured glazing, communicated to the Royal Society by Mr Heinius of Petersburg. Take of litharge three parts, of sand or calcined flint one part; mix these very well together, then run them into a yellow glass with a strong fire. Pound this glass, and moisten it when in subtile powder with a well-saturated solution of silver, which make into a paste. Put this paste into a crucible, and cover it. Give at first a gentle degree of fire, then increase and continue it till you have a perfect glass which will be green. Pound this glass again, and reduce it to a fine powder: moisten this powder with some beer, so that by means of an hair-pencil you may apply it upon the vessels. Those 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, and take out the vessels. This prepara-
tion owes its colour to the silver in it; the precipitates of which, spread upon a glass plate, have the property of staining it yellow by ignition, without fusion. When used in glazing vessels, however, they must be held over the smoke of burning vegetables, in order to call forth the beautiful colour.
A red or green tinge may be given to glazings by means of copper. The red colour appears first; but by a continuation of the fire, it is changed into a green. The finest red colour, however, that can be given to glazings, is that prepared from the solution of gold in aqua regia, as mentioned under the article GLASS.—The finest blue is always given by means of zaffre or smalt. A purplish colour, as well as a brown or black, may also be given by manganese; but a mixture of the materials for red and blue will undoubtedly produce the finest colour. The colouring materials for glazings, therefore, in short are the following.
Red—Gold or copper.
Yellow—Silver, iron.
Green—Copper.
Blue—Zaffre or smalt.
Black—Manganese.
White—Calx of tin.
Each of these materials mixed up in a proper quantity with any composition that readily vitrifies, will form a glazing of the desired colour upon any kind of earthen ware. The proportions in which they are to be used may easily be determined by a few trials.—Stone ware is glazed by another method, viz. the vitrification of a small part of the substance of the ware itself, by the fumes of salt thrown into the furnace when the vessels are intensely heated. See STONE WARE.—The application of different colours to it, however, is equally easy with the former.
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 liable to any of the accidents which may ensue from the use of vessels glazed with lead.