It is then placed in a press and subjected to enormous pressure from a steel die. This reduces the powder to a third of its bulk and thoroughly consolidates it; at the same time the pattern, if any, is impressed upon the tile by means of the die.
1 Report on International Exhibition 187]; by Gilbert Redgrave, Esq.
They are then carefully dried in a hot room, glazed, and fired.
There are several places in which encaustic tiles are made, but the most celebrated manufactory in the country is that of Messrs. Minton, Hollins, & Co., at Stoke-upon-Trent; the founder of which, Mr. Herbert Minton, brought the art to its present state of perfection.
There are other manufacturers at Stoke, Staffordshire, at Poole in Dorsetshire, also at Broseley, near Hereford.
Tesserce are tiles sometimes made by the dry process just described, and are so accurate in form that they can be laid as mosaic work in pavement without any rubbing or injury of the face.
They are sometimes made out of moist clay, and cut into various shapes by wires.
Majolica Tiles have raised patterns, and their colour "applied in the form of an enamel or coloured opaque glaze. They have not therefore the same amount of durability, and are only used for walls and similar ornamental purposes."1
Flat encaustic tiles made by either process may be used for paving or wall decoration, but those with raised patterns must of course be restricted to the latter purpose.
In some cases the tiles for wall decoration are put together in panels before being glazed. A picture is painted upon the panel, the tiles composing it are then separated, burnt at a high temperature, and glazed.
In order to make these Notes more complete, and as a matter of interest to those who possess the necessary chemical knowledge, a description of the analysis of a hrick earth or brick is appended.3
1 Report on the International Exhibition, 1871; by Gilbert Redgrave, Esq.
2 Gwilt's Encyclopaedia. 3 From Notes on the Chemistry of Building Materials, by Captain Abney, R.E., F.R.S.
"The analysis of a brick or brick earth is conducted in a very similar manner to that of a lime or a cement (see p. 239).
"To find the constituents, a small portion of the finely powdered brick is taken and treated with dilute HC1, and digested with it until nothing more is dissolved. The solution is then filtered and tested in the same manner as directed when treating on cements.
"The insoluble residue, after being dried, is mixed with about three times its bulk of a mixture of the carbonates of soda and potash, in the proportion of 51/2 parts of the sodium to 7 of the potassium carbonate, and placed in a platinum crucible containing a little of the fluxing mixture at the bottom, and heated over a strong flame. The mass will melt and a decomposition take place, owing to the silicic acid of the brick having, when heated, a stronger affinity for the soda and potash of the fluxing mixture than for the bases with which it was combined in the brick.
"The crucible should be allowed to remain over the flame till no more bubbling is observed, caused by the escape of C02, which will generally be in about five or ten minutes; it should then be carefully removed and allowed to cool, and the contents as far as possible detached from the sides of the crucible, and the whole placed in a dish with some dilute HC1 which will dissolve the fused mass. The crucible should be removed from the dish, and, after washing it, the solution should be evaporated to complete dryness, re-dissolved in acid, and the Si02, which by the process of drying has become insoluble, be filtered out. The examination of the solution should be proceeded with as the one originally obtained, after treating the brick with acid.
"The common constituents of bricks are:
Silica. Manganese. Lime. Soluble Salts.
Iron. Alumina. Magnesia. Sulphur.
The sulphur may exist in the brick in one or two states, either as sulphuric acid, or combined with iron as sulphide of iron. If contained in the acid state it may be detected by boiling some in water and adding barium chloride, which produces a white precipitate insoluble in acids. As a sulphide it may be detected by fusing a portion with a little of the fluxing mixture, detaching the fused mass and moistening it on a silver plate or coin. The sulphide of soda formed by the fusion will blacken or stain the silver.
"Quantitative Analysis. - 25 grains are very carefully weighed out and transferred to a porcelain dish into which dilute hydrochloric and nitric acids are poured, and the whole heated; the portion remaining undissolved is filtered out, dried, and weighed. The nitric acid acts upon any sulphur which the brick may contain and converts it into sulphuric acid."