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long, stroke it every way, that there be no crease or wrinkles in it; then drop a little mercury upon it, and with a piece of cotton, wool, or hair's foot, spread it all over the foil, so that every part may be touched with the mercury. Then, keeping the marble slab nearly level with the horizon, pour the mercury over the foil ; cover it with a fine paper ; and lay two weights very near its lowest end or side, to keep the glass steady, while you draw the paper from between the silvered foil and the glass, which must be laid upon
paper, you must take care that no air bubbles be left; for they will always appear, if left in at the first. You must likewise be sure to make the glass as clean as possible on the side intended to be silvered, and have the
paper also quite clean; otherwise, when you have drawn the paper from under it, dull white streaks will appear, which are very disagreeable.
After the paper is drawn out, place as many weights upon the glass as you conveniently can, in order to press out the superfluous mercury, and make thé foil adhere to the glass. When it has lain six or seven hours in this situation, raise the stone about two or three inches at its highest end, that as much of the mercury may run off as possible ; let it remain two days before you venture to take it up. But before you take the weights off, gently brush the edges of the glass, that no mercury may adhere to them, then take it up, and turn it directly over, with its face side downward; but raise it by degrees, that the mercury may not drip off too suddenly: for if, when taken up, it is immediately set perpendicular, air will get in between the foil and the glass at the top, as the mercury descends to the bottom; by which means, .
if you be not exceedingly careful, your labour will be lost.
Another method, is to slide the glass over the foil, without the assistance of
To Silver Glass Globes.
Take half an ounce of clean lead, and melt it with an equal weight of pure tin; then immediately add half an ounce of bismuth, and carefully skim off the dross; remove the mixture from the fire, and, before it grows cold, add five ounces of mercury, and stir the whole well together; then put the fluid amalgam into a clean glass, and it is fit
When this amalgam is used for foiling or silvering, let it first be strained through a linen rag ; then gently pour some ounces of it into the globe intended to be foiled: the mixture should be poured into the globe, by means of a glass or paperfunnel, reaching almost to the bottom of the globe, to prevent its splashing to the sides; the globe should then be dexterously inclined every way, though very slowly, in order to fasten the silvering. When this is once done, let the globe rest some hours ; repeat the operation, till at length the fluid mass is spread even, and fixed over the whole internal surface, as it may be known to be, by viewing the globe against the light; the superfluous amalgam may then be poured out, and the outside of the globe cleared.
To Silver the Convex Side of Glasses for Mirrors,
Take an earthen plate, on which pour some prepared plaster of Paris, mixed with water, of a
proper consistence; then immediately, before it grows too stiff, lay the glass, with its convex side downward, in the middle of the plate, and press it until it lies quite close to the plaster; in which situation let it remain until the plaster becomes quite dry. After which, work a groove with your finger, round the outside of the glass, in order to let the superfluous mercury rest upon it ; then cut the tinfoil to a proper size, and press it with the glass into the plaster-mould, in order to make it lie close; after which, cover it with the mercury, and, without a paper (as directed for sil. vering plain mirrors), slide it over the silvered foil; then place a weight on it, and let it stand two or three days, rising it by degrees, that the mercury may drip off gradually.
After this method common window-glass, &c. may be silvered.
To lay Paper Prints on the Inside of Glass Globes.
First, cut off all the white part of your impression, so that nothing appear but the print; then prepare some strong gum arabic water, or size, with which you must brush over the face side ; after which put it into the globe, and with a long small stick, on which a camel's-hair pencil is fixed, stick it even on; and by this method you may put what number of prints you please into the globe. Let them dry about twelve hours; then pour some prepared plaster of Paris, either white or tinged, whatsoever colour you please, and turn the globe easily about, so that every part be covered; pour out the superfluous plaster, and it is finished.
Tinning is the art of covering any metal with a thin coating of tin. Copper and iron are the metals most commonly tinned. The use of tinning these metals is to prevent them from being corroded by-rust, as tin is not so easily acted upon by the air or water as iron and copper are.
What are commonly called tin-plates, or sheets, so much used for utensils of various kinds, are in fact iron-plates coated with tin.
The principal circumstance in the art of tinning, is to have the surfaces of the metal to be tinned perfectly clean and free from rust, and also that the melted tin be perfectly metallic, and not covered with any ashes or calx of tin.
Tinning of Iron. When iron-plates are to be tinned, they are first scoured, and then put into what is called a pickle, which is oil of vitriol diluted with water ; this dissolves the rust or oxyd that was left after scouring, and renders the surface perfectly clean. They are then again washed and scoured. They are now dipped into a vessel full of melted tin, the surface of which is covered with fat or oil, to defend it from the action of the air. By this means, the iron coming into contact with the melted tin in a perfectly metallic state, it comes out completely coated.
When a small quantity of iron only is to be tinned, it is heated, and the tin rubbed on with a piece of cloth, or some tow, having first sprinkled the iron with some powdered resin, the use of which is to reduce the tin that may be oxydated. Any
inflammable substance, as oil, for instance, will have in some degree, the same effect; which is owing to their attraction for oxygen.
The Tinning of Copper. Sheets of copper may be tinned in the same manner as iron. Copper boilers, saucepans, and other kitchen utensils, are tinned after they are made. They are first scoured; then made hot; and the tin rubbed on, as before, with resin. Nothing ought to be used for this purpose but pure grain-tin; but lead is frequently mixed with the tin, both to adulterate its quality, and make it lay on more easily; but it is a very pernicious practice, and ought to be severely reprobated.
To whiten Brass or Copper by boiling. Put the brass or copper into a pipkin with some white tartar, alum, and grain-tin, and boil them together. The articles will soon become covered with a coating of tin, which, when well polished, will look like silver. It is in this manner that pins, and many sorts of buttons, are whitened.
BRONZING. Bronzing is colouring plaster, or other busts and figures, with metallic powders, in order to make them appear as if made of copper or other metals. The powders used for this purpose are either fine copper-filings, aurum musivum, or copper precipitated from its solution in aqua fortis by iron. Having done over the substance to be bronzed with a dark green colour, the projecting parts are touched with either isinglass size, japanners' gold size, or, in some cases, with drying-oil, or oil-paint; the powders are