Mirror Glass

“Mirror Glass after it comes from the factory, goes into the hands of the grinder and silverer, that it may be completed for framing. Before they begin to grind, they bed the glass in plaster of Paris, spread upon a table made on purpose. Then they take sand and water, and, with another glass of smaller size they rub it to a perfectly even surface, which then has the appearance of smooth slate. 

The next operation, is what the call mirror smoothing, which is preparative to polishing; to effect, they lay a wet blanket on a stone table, and taking water and emery of different degrees of fineness, with another glass, as before in grinding, they skue it until it is smooth, and then proceed to polish it. For this purpose, they bed it again in the plaster of Paris, and take an oblong block of wood, covered in course blanket or lap, as they term it, charged with tripple, which is made of iron ore. This block has to stick fixed to it, the upper end of which is fastened to the floor above, and this stick giving way as a bow, it presses the block every way upon the glass. as it is moved backwards and forewards on its surface. When they have done with this block they put to another charged with putty, made from grain tin, with which they rub as before, and bring it to a fine transparent polish ready for silvering. This last process, they term white lapping. They then prepare the glass for silvering, by taking dry whitening, and with a hand rub it about the glass, to take off the dampness, and cleanse it from spots, that it may receive the quicksilver (mercury). On a table of stone, they lay a sheet of tin foil, which must be as large as the plate of glass; and to make the quicksilver adhere to the foil they first rub a quantity of quicksilver, with the hand over the foil to cleanse it. 

This they term quicksilvering the foil. After this, they pour on to the foil, a proper quantity of quicksilver sufficient to cover all the foil and flow over the mirror glass. Then they take clean cloths to clear the mirror plate from all dust before they lay it on the silver, which they do by shutting it over the edge of a paper doubled on one edge of the foil, till the glass comes to its due place on the silver. This method prevents the air from being drawn in between the mirror plate and the silver, which would prevent the silver from fixing. The glass is then pressed down to the quicksilver, by a number of lead weights, that the quicksilver may adhere to the glass, and the foil also. In this state it is to remain a few hours or days, according to the size of the glass. A glass 18 by 30 inches, requires 5 or 6 hours, and one of 60 by 100, 5 or 6 days under the weights. And after the glass is turned over, it requires to lay a few days in a sloping direction , that the unfixed quicksilver may descend to one corner of the glass, and so fall off. 

Mirror Glasses for chimney pieces run various, according to the size of the fire-place, and the height of the wall above. To save expence, they are sometimes fitted up in three plates, and the joints of the glass covered with small gilt mouldings or pillasters. At other times with the naked joint only. When they are managed in this way, the expense of the plate is reduced to one third less, or more sometimes. It adds however something to the expense of the frame, but not always; for when they are of one plate the frame in general is made bolder and more elegant. 

Glasses may, however by ordered to any size to suit the pier they are for, from 36 by 60 inches to 75 by 117, which is the largest they cast, at the Brtitish Factory, Blackfriars Bridge, London. 

On this article, I have received my information from Mr. Black, Glass Cutter, near the Seven Dials, Long Acre; so that the preceding particulars on grinding, polishing and silvering may be depended upon as genuine.” 

Thomas Sheraton 1803

Finished mirror plates were typically fitted to carved and gold leaf water gilt regence, rococo, neo-classical, and Empire frames.

18th-century glass mirror workshop from Diderot’s Encyclopedia


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