From this description it will be seen, that the various colours in the different parts of the window are put in as pieces, and that no colours, properly so called, are applied by the brush to the surface. There are, however, certain tints of the "tracing brown," which can be obtained by the addition of black oxide of manganese, or by a different method of preparation of the oxide of iron, to give it its body. Sulphate of iron, when heated, loses its sulphuric acid, and the oxide, which was, as sulphate, in the state of protoxide, becomes, by heating, the red or peroxide of iron; its tint, when made in this way, being generally lighter than the tint of that form of oxide which is employed as ordinary tracing brown. It is sometimes called flesh tint, though this is decidedly an objectionable name for it.
It has been suggested to me, that I should give some receipts for the manufacture of the enamel colours used in mediæval glass painting; I have therefore added a few which are easily prepared. Others of a more complicated nature had much better be obtained from the makers of the enamel used in porcelain painting. And here again, let me remark, that in ordering fluxes from these manufacturers, it should be stated especially that a flux is required which does not contain borax, nor should the painters in any establishment be allowed to use these softer fluxes, which they are almost certain to do, unless forbidden; for though they are easier to work with, they will infallibly lead to calamitous results.
| Yellow. | ||
| Oxide of tin | 2 | parts. |
| Oxide of antimony | 2 | " |
| Red Lead | 16 | " |
| Orange. | ||
| Red Lead | 12 | " |
| Oxide of antimony | 4 | " |
| Persulphate of iron | 1 | " |
| Flint powder | 3 | " |
| Brown. | ||
| Black oxide of manganese | 2·25 | " |
| Flint slate (powdered) | 4·0 | " |
| Red lead | 8·5 | " |
| Brown Red. | ||
| Crocus (oxide of iron) | 3 | " |
| Green sulphate of iron (calcined) mixed with six parts of these No. 2. | 1 | " |
| Light Red for Flesh Tints. | ||
| Carbonate of lead | 1·5 | " |
| Persulphate of iron (calcined) | 1 | " |
| Flint glass | 3 | " |
The use of enamels—that is, substances which impart various colours to the glass, when placed on its surface by their fusion—is not admissible in windows which pretend to belong to any of the earlier styles of glass painting; though enamel painting is used for the decoration of houses, and sometimes, as I consider very improperly, for the decoration of church windows. One sheet of glass, colourless and transparent, or it may have its surface ground, is usually employed. A subject is painted on it with enamel colours, much as subjects are painted upon porcelain. When the work is completed, the glass plate is fired, and thus the colours become semi-transparent, and perfectly adherent to the plate; but they are not clear and bright, and transparent, as are the colours of glass which is coloured in the pot, and therefore have not the same brilliancy, nor do they allow of the same bold and effective treatment.
It is much to be desired that amateurs who can draw, and who have a feeling for this particular style of art, should devote a portion of their time to its execution. They will find it to be extremely agreeable and pleasant, and the few difficulties which they meet with in their first attempts will be readily overcome by perseverance, or by applying for assistance and advice to gentlemen engaged in the pursuit of this interesting profession.
Moulded and Cut Glass.—Flint glass is now very commonly blown in moulds, and this art has been brought to such perfection that moulded decanters and tumblers have an appearance very similar to that of cut glass. The moulds are always made of metal, and so constructed, that they open out into two or more pieces, which are generally hinged to the bottom of the mould. The workman places it on the ground, and fixes it by standing on projections from its side. He then gathers a suitable quantity of glass on the end of his blowpipe, which he places in the mould, and the side of the glass touching it will thus have impressed upon it whatever form is engraved on it. After the glass has become hard, the mould is opened, and the glass vessel is removed and annealed.
When it is desired to cut a design on the outside of a tumbler or wine-glass, the vessel is, in the first instance, blown of a thicker substance than if it is to be left uncut. The necessary shapes, which are usually in facets, are cut upon it by the action of sand and water, a lathe of a very simple construction being used to give a rotary motion to cutting discs, made of stone and kept continually moist by water dripping on them, so that when the glass is pressed against them, the required portion of its surface is worn away. The usual diameter of these stones is about 10 inches. After the rougher stone has been used, a finer kind of sandstone disc is employed, or a disc of slate, upon which sand and water are allowed to drop, and the already roughly cut surface is, by their action, partly polished. Copper discs with flattened circumference are used for polishing the glass, and for this purpose, emery mixed with oil, is applied to the edges of their circumference.
Ground Glass is made by rubbing the surface of glass with sand and water, just as in the first operation of plate glass polishing. But a very ingenious method is now generally adopted for grinding glass, by placing it in a cradle, so that it can swing from side to side; sand and water are placed upon the glass, and it grinds itself, so to speak, by this operation.
Annealing and Devitrification.—As the word "annealing" has been often used in this article, it will be well to explain what is its action. If a piece of molten glass be dropped into water, it will assume an oblong shape, the lower end of which will be round, while the other will taper off into a fine point. These drops, which have received the name of Prince Rupert's drops, look like pieces of ordinary glass, and if the small end of one of them be broken off, a sort of explosion takes place, and the whole mass flies into a thousand minute pieces, some of which will be found to have been driven to a considerable distance. Here then it appears, that when the skin, which is perfect and entire in the Rupert drop, is broken, the bond which held together the constituent particles is broken also, and so they are acted on by a repellent force, and fly away from one another. If hot water be poured into a thick common tumbler, it very generally cracks it: but if the tumbler be thin and of better manufacture, it will bear almost boiling water without cracking. In the first case it has been badly annealed; and besides this, glass being a bad conductor of heat, from its thickness, the heat imparted by the hot water expands the inner surface, while the outer coating, not being warmed, does not expand, and, retaining its original form, is burst. If, however, a tumbler be thick and properly annealed, there is not so much danger of its breaking, when a portion of it is exposed to a considerable rise of temperature. In the case of the Rupert drops, they are not annealed at all, and so there is no cohesive bond between the particles, such as there would be if they were properly annealed, that is, if, instead of being cooled suddenly from the molten state, they were allowed to cool in a heated chamber very slowly. After glass has been heated, the particles of which it is composed take a long time to rearrange themselves, so that in the manufacture of thermometers, it is necessary, after sealing up the bulb and tube which contain the mercury, to allow them to remain for a long time; otherwise the pressure of the air on the outside of the bulb, not being supported by any air on the inside, causes the particles of glass to become more compact, and thus renders the capacity of the thermometer bulb and tube smaller than it was, when the thermometer was first sealed. It seems that the process of annealing glass gives time for the particles to arrange themselves in such a way, that when the glass is cold, it will not be so liable to fracture from sudden changes of temperature.
Considerable curiosity has been excited of late by a new invention, which has resulted from the investigations of a Frenchman. We have been told that tumblers and wine-glasses, and other glass utensils, could be so treated that they would never break; and experiments performed upon many samples of these glasses led one to suppose, that the object had been attained. There is no doubt whatever, that some who have had experience of what is termed toughened glass know, that in many cases very uncertain results are obtained in the resisting power of the glass to the action of a violent blow. Before, however, entering into some researches which I have made on the subject, it will be well to state what is the nature of the change which the toughening process produces in the glass, and this seems to be a fit place for this consideration, as the method of making, and the behaviour, of Prince Rupert's drops, have just been discussed.