HOW TO PREPARE GOLD FOR USE IN PHOTOGRAPHY.
Several queries have been sent to the editor recently asking how to prepare gold for photographic use. Gold is one of the chemical elements. Its symbol is "Au," the first two letters of the word aurum, the Latin name for gold. Gold is used in photography in the form of chloride of gold. To make chloride of gold, pure gold is dissolved in a mixture of nitric and hydrochloric (muriatic) acid. This mixture is called "aqua-regia," from its being the only known solvent of gold. It is made by mixing one part of nitric acid, two parts of muriatic acid, and three parts of water. Gold dissolves very readily in this mixture.
Chloride of gold may be made from gold-leaf (such as is used by dentists), gold coins, scraps of gold ornaments, etc. Where the amateur prepares his own gold about half the expense is saved. Put the gold into a glass vessel and pour over it eight times its weight of aqua-regia. Set the vessel in a dish of hot water, and let it stand on the back of the stove till the gold is entirely dissolved. Pour the solution into a porcelain crucible, and subject to heat till all the free acid is evaporated or driven off. After the acid is evaporated, add three or four drachms of distilled water and evaporate again. When the water is evaporated, enough distilled water must be added to make the solution up to a standard strength—one grain of gold to three drachms of water. If twenty-four grains of pure gold are used, add nine ounces of distilled water. Keep this solution in a dark place or in an opaque bottle. The bottle may be wrapped in black needle-paper, which will also protect it from the light.
Gold coins and jewelry contain more or less alloy, but this does not seem to affect the print in any way. One grain of gold will tone from twenty to twenty-five cabinet prints. The chemical formula for chloride of gold is AuCl3, meaning that a molecule of chloride of gold contains one atom of gold and three atoms of chlorine. In order to preserve the gold chloride longer, it is usually prepared with salt, and is called chloride of gold and sodium. It is in this form that it is sold for use in photographic work, the pure chloride of gold attracting and absorbing moisture from the air.
The chloride of gold and sodium is prepared by dissolving common salt in a solution of chloride of gold and then evaporating the solution. Sodium chloro-aurate is also another name for this salt. Chloride of sodium is common salt, and the chemical formula is NaCl, meaning that it is composed of one part natrium (the Latin name for sodium) and one part chlorine. The chemical formula for chloride of gold and sodium is NaCl, AuCl3+2H2O, meaning that it is composed of one molecule of chloride of sodium, one molecule of chloride of gold, to which are added (+) two molecules of water. The chemical formula is also written in this way: NaAuCl4+2H2O. When chloride of gold and sodium is used for toning, a larger quantity by weight must be used than when the pure chloride of gold is used.
A stock solution may be prepared by adding 15 grains of chloride of gold and sodium to 7½ oz. of water. (By a "stock solution" is meant a solution that keeps for a long time, and may therefore be prepared in a large quantity.)
The toning-bath is made by taking 3½ oz. of water and pouring in the gold solution till the mixture will turn blue litmus-paper red. (About half an ounce will be sufficient.) To this mixture add bicarbonate of soda until it turns the red litmus back to blue. This bath should be prepared about half an hour before it is needed for toning. A saturated solution (see first paper on simple chemistry) should be made of bicarbonate of soda, and kept in stock.
Bicarbonate of soda is a fine white powder, soluble in ten parts of water. It is used for neutralizing the excess of acid in gold toning-baths. Natural deposits of bicarbonate of soda are found in Africa, where it is called "trona," and in South America, where it is called "urao." Its chemical formula is HNaCO3.]
Names of chemical elements mentioned and their atomic weight:
| Atomic | ||
| Chemical Element. | Symbol. | Weight. |
| Carbon | C | 12 |
| Chlorine | Cl | 35.5 |
| Gold (Latin name Aurum) | Au | 196 |
| Hydrogen (standard weight) | H | 11 |
| Oxygen | O | 12 |
| Sodium (Latin name Natrium) | Na | 23 |
Hydrogen is the lightest substance known, and an atom of hydrogen is used as the standard weight by which all other atoms of the chemical elements are weighed.
Sir Knight Silas Leon Smith, New Orleans, La., asks for a formula for making paper which can be exposed in the camera like a plate. Calotype-paper is probably the paper which Sir Silas says he has seen described, and which produces a positive picture when exposed in the camera. The process is too long to describe in the space devoted to the "Answers to Querists," but the formula may be found in Wilson's Cyclopedia of Photography, which is in most public libraries. Sir Silas sends a formula for sensitizing paper to produce a red image, for which he will please accept thanks. The formula will soon be published in the Camera Club, and credit given.
Sir Knight Frank Evans, Jun., sends the following formula for developer, which he recommends both for plates and for bromide paper.
EIKONOGEN DEVELOPER.
NO. 1.
Sulphite of Soda (Crystals) 3 oz. Hot Water 45 " Thoroughly dissolve, then add 1 oz. of eikonogen.
No. 2.
Sal Soda 4 oz. Water 15 " To develop, take of No. 1, 3 oz.; No. 2, 1 oz.
This developer can be used over again.
| Sulphite of Soda (Crystals) | 3 | oz. |
| Hot Water | 45 | " |
| Sal Soda | 4 | oz. |
| Water | 15 | " |