Preparation of the mercurial solution

The amalgam of gold is applied to bra , through the intervention of pure nitric acid, holding in solution a little mercury. 100 parts of mercury, and 110 parts by weight of pure nitric acid, specific gravity 1·33, are to be put into a gla matra . On the application of a gentle heat the mercury di olves with the disengagement of fumes of nitrous gas, which must be allowed to escape into the chimney. This solution is to be diluted with about 25 times its weight of pure water, and bottled up for use. 1. Annealing. —The workman anneals the piece of bronze after it has come out of the bands of the turner and engraver. He sets it among burning charcoal, or rather peats, which have a more equal and lively flame; covering it quite up, so that it may be oxidized as little as po ible, and taking care that the thin parts of the piece do not become hotter than the thicker. This operation is done in a dark room, and when he sees the piece of a cherry red colour, he removes the fuel from about it, lifts it out with long tongs, and sets it to cool slowly in the air. 2. The dec a page .—The object of this proce is to clear the surface from the coat of oxide which may have formed upon it. The piece is plunged into a bucket filled with extremely dilute sulphuric acid; it is left there long enough to allow the coat of oxide to be di olved, or at least loosened; and it is then rubbed with a hard brush. When the piece becomes perfectly bright, it is washed and dried. Its surface may however be still a little variegated; and the piece is therefore dipped in nitric acid, specific gravity 1·33, and afterwards rubbed with a long-haired brush. The addition of a little common salt to the dilute sulphuric acid would probably save the use of nitric acid, which is so apt to produce a new coat of oxide. It is finally made quite dry, (after washing in pure water) by being rubbed well with tanners’ dry bark, saw-dust, or bran. The surface should now appear somewhat de-polished; for when it is very smooth, the gold does not adhere so well. 3. Application of the amalgam. —The gilder’s scratch-brush or pencil, made with fine bra wire is to be dipped into the solution of nitrate of mercury, and is then to be drawn over a lump of gold amalgam, laid on the sloping side of an earthen ve el, after which it is to be applied to the surface of the bra . This proce is to be repeated, dipping the brush into the solution, and drawing it over the amalgam, till the whole surface to be gilded is coated with its just proportion of gold. The piece is then washed in a body of water, dried, and put to the fire to volatilize the mercury. If one coat of gilding be insufficient, the piece is washed over anew with amalgam, and the operation recommenced till the work prove satisfactory. 4. Volatilization of the mercury. —Whenever the piece is well coated with amalgam, the gilder exposes it to glowing charcoal, turning it about, and heating it by degrees to the proper point; he then withdraws it from the fire, lifts it with long pincers, and, seizing it in his left hand, protected by a stuffed glove, he turns it over in every direction, rubbing and striking it all the while with a long-haired brush, in order to equalize the amalgam. He now restores the piece to the fire, and treats it in the same way till the mercury be entirely volatilized, which he recognises by the hi ing sound of a drop of water let fall on it. During this time he repairs the defective spots, taking care to volatilize the mercury very slowly. The piece, when thoroughly coated with gold, is washed, and scrubbed well with a brush in water acidulated with vinegar. If the piece is to have some parts burnished, and others dead, the parts to be burnished are covered with a mixture of Spanish white, bruised sugar-candy, and gum di olved in water. This operation is called in French epargner ( protecting ). When the gilder has protected the burnished points, he dries the piece, and carries the heat high enough to expel the little mercury which might still remain on it. He then plunges it, while still a little hot, in water acidulated with sulphuric acid, washes it, dries it, and gives it the burnish. 5. The burnish is given by rubbing the piece with burnishers of hematite (bloodstone). The workman dips his burnisher in water sharpened with vinegar, and rubs the piece always in the same direction backwards and forwards, till it exhibits a fine polish, and a complete metallic lustre. He then washes it in cold water, dries it with fine linen cloth, and concludes the operation by drying it slowly on a grating placed above a chafing dish of burning charcoal. 6. The deadening is given as follows. The piece, covered with the protection on those parts that are to be burnished, is attached with an iron wire to the end of an iron rod, and is heated strongly so as to give a brown hue to the epargne by its partial carbonization. The gilded piece a umes thus a fine tint of gold; and is next coated over with a mixture of sea salt, nitre, and alum, fused in the water of crystallization of the latter salt. The piece is now restored to the fire, and heated till the saline crust which covers it becomes homogeneous, nearly transparent, and enters into true fusion. It is then taken from the fire and suddenly plunged into cold water, which separates the saline crust, carrying away even the coat of epargne . The piece is lastly pa ed through very weak nitric acid, washed in a great body of water, and dried by exposure either to the air, over a drying stove, or with clean linen cloths. 7. Of or-moulu colour. —When it is desired to put a piece of gilded bronze into or-moulu colour, it must be le scrubbed with the scratch-brush than usual, and made to come back again by heating it more strongly than if it were to be deadened, and allowing it then to cool a little. The or-moulu colouring is a mixture of hematite, alum, and sea salt. This mixture is to be thinned with vinegar, and applied with a brush so as to cover the gilded bra , with reserve of the burnished parts. The piece is then put on glowing coals, urged a little by the bellows, and allowed to heat till the colour begins to blacken. The piece ought to be so hot that water sprinkled on it may cause a hi ing noise. It is then taken from the fire, plunged into cold water, washed, and next rubbed with a brush dipped in vinegar, if the piece be smooth, but if it be chased, weak nitric acid must be used. In either case, it must be finally washed in a body of pure water, and dried over a gentle fire. 8. Of red gold colour. —To give this hue, the piece after being coated with amalgam, and heated, is in this hot state to be suspended by an iron wire, and tempered with the composition known under the name of gilder’s wax; made with yellow wax, red ochre, verdigris, and alum. In this state it is presented to the flame of a wood fire, is heated strongly, and the combustion of its coating is favoured by throwing some drops of the wax mixture into the burning fuel. It is now turned round and round over the fire, so that the flame may act equally. When all the wax of the colouring is burned away, and when the flame is extinguished, the piece is to be plunged in water, washed, and scrubbed with the scratch-brush and pure vinegar. If the colour is not beautiful, and quite equal in shade, the piece is coated with verdigris di olved in vinegar, dried over a gentle fire, plunged in water, and scrubbed with pure vinegar, or even with a little weak nitric acid if the piece exhibit too dark a hue. It is now washed, burnished, washed anew, wiped with linen cloth, and finally dried over a gentle fire. The following is the outline of a complete, gilding factory, as now fitted up at Paris. IMG:4147767755307473660_illo0616.png:Gilding workshop Fig. 529. Front elevation and plan of a complete gilding workshop. P. Furnace of appel , or draught, serving at the same time to heat the deadening pan ( poêlon au mat ). F. Ashpit of this furnace. N. Chimney of this furnace constructed of bricks, as far as the contraction of the great chimney S of the forge, and which is terminated by a summit pipe rising 2 or 3 yards above this contraction. B. Forge for annealing the pieces of bronze; for drying the gilded pieces, . C. Chimney of communication between the annealing forge B , and the space D below the forge. This chimney serves to carry the noxious fumes into the great vent of the factory. U. Bucket for the brightening operation. A. Forge for pa ing the amalgam over the piece. R. Shelf for the brushing operations. E E. Coal cellarets. O. Forge for the deadening proce . G. Furnace for the same. M. An opening into the furnace of appel , by which vapours may be let off from any operation by taking out the plug at M . I. Cask in which the pieces of gilded bra are plunged for the deadening proce . The vapours rising thence are carried up the general chimney. J J. Casement with gla panes, which serves to contract the opening of the hearths, without obstructing the view. The casement may be rendered movable to admit larger objects. H H. Curtains of coarse cotton cloth, for closing at pleasure, in whole or part, one or several of the forges or hearths, and for quickening the current of air in the places where the curtains are not drawn. Q. Opening above the draught furnace, which serves for the heating of the poêlon au mat (deadening pan).