Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XXX.
POSITIVE PRINTING.

Printing on Plain Paper, on Albumenized Paper, or Arrow-Root Paper.

THE theory and practice of positive printing are second only in time, not in importance, to the theory and practice of the negative; it is rare, however, that the same amount of care and labor is bestowed upon this department as upon that of taking a negative. We run all sorts of risk, make every effort, incur immense expenses in order to secure a first-rate negative, and then frequently abandon the gem into the hands of an indifferent assistant, which is tantamount in many instances to leaving the negative to print itself. What an analogy exists here between that of planting and cultivating; that of begetting and of educating ! Do not some farmers dibble a hole, insert the seed, and then conclude their labor is ended? Do not some parents almost come to the same conclusion? They both leave the cultivation and education of the young germs to the sun, the wind, and the weather, not to Providence; for he that believes in Providence, puts his shoulder to the wheel and works for Providence. In a manner quite analogous, the photographer neglects the execution of the printing department, regards the operation as secondary, concludes that having secured a good negative, prints will grow from it like potatoes from the seedling. This negligence must be abandoned, and more vigorous action commenced.

Positive printing is two-fold, consisting in direct printing by the rags of the sun, and printing by development or continuation; in the former case the image becomes visible during the operation by means of light itself; in the latter case the impression made by light is latent, and is rendered visible afterward by chemical reduction. The chemical materials used in the preparation of the paper for the reception of the image are, first, surface materials for communicating a more uniform and smooth layer, such as albumen, gelatine, starch and gums; secondly, substances that undergo some physical or chemical change by the agency of light, and which are mixed with the surface-materials; these are the chlorides, bromides and iodides of the various metals. Paper, so prepared, is sensitized in the dark-room in a bath of nitrate of silver; the chloridized paper, when sensitized, yields an image by the direct operation of light. Paper, prepared with the other salts, receives an invisible impression of the image, which is made manifest in a bath of gallic acid or some other material, according to the circumstances of the case. The image obtained by the direct agency of light has a beautiful color, but the picture is not permanent, for light continues still to act upon the prepared film, and finally obliterates the image. The positive thus obtained, therefore, has to be fixed in the same manner as the collodion picture, and by one of the same fixing solutions, hyposulphite of soda. But the color of the image after fixation is far from being bright and agreeable; we have, therefore, to resort to means before fixing, during fixing or afterward, by which the color can be restored, or an agreeable color can be communicated. This operation is denominated the toning of the picture. The chemical substances used in this operation are: chloride of gold, and sometimes nitrate of uranium, together with certain accessories that modify the action of these two salts, such as carbonate of soda, carbonate of lime, phosphate of soda, acetate of soda, chlorinetted lime, citrate of soda, etc. Direct positive printing will occupy our attention first. The subject is divisible into the following branches; Description of the principal materials used; Preparation of the paper; Sensitizing of the paper; Printing by exposure to the sun; Washing of the prints; Toning of the prints; Fixing of the prints; Washing of the fixed prints; Drying of the prints; Cutting and Mounting of the prints.

Description of the Materials used in Positive Printing.

Paper, suitable for photographic purposes, must be homogeneous throughout, and of a very fine texture. The surface particularly must be uniform and satinized, free from all marks or specks, or chemical particles which, by decomposition afterward, would spoil the picture. Such paper can be had of the different photographic establishments, from the various paper-mills of America, England, France, Germany, etc. Owing to the different materials employed in the sizing of the paper arises a difference in the tone of the photographic picture; some sizing consists of starch, others of gelatine.

Albumen.

This substance derives its name from the white of egg, of which it constitutes the greatest quantity. It is found also in blood, in the form of serum, (the fluid in which the blood corpuscles swim,) in the serum of milk, in all serous secretions, etc. It exists in two forms, soluble and insoluble. When coagulated, or in the insoluble form, it constitutes a portion of most of the solid tissues of the animal frame. Solid albumen can be obtained by evaporating either the serum of blood, (the watery fluid which separates from the clot after coagulation,) or the white of an egg to dryness, at a temperature not exceeding 120°. The latter substance must first be broken up thoroughly, so as to separate the membranous or fibrous material that holds it together in a compact form, and then after subsidence the fluid portion is decanted. The dry mass is a yellow, transparent, tough and hard substance, consisting of albumen, with a small quantity of the saline substances that exist in this material, and which may be separated by digestion in alcohol and ether. So dried, it swells up when put in water and finally dissolves. Before it is dissolved, it, may be heated to a higher temperature than the boiling point of water before it passes into the insoluble condition; but when dissolved in water and heated to a temperature between 140° and 150°, it coagulates, and becomes quite insoluble in water. Albumen in solution is precipitated by alcohol, acids, metallic salts, and several organic bodies, such as tannic acid and kreosote. The precipitates of albumen by metallic salts constitute two distinct substances, namely, albumen with the acid, and albumen with the oxide, of which generally the former is soluble and the latter insoluble. Pure albumen is supposed to be really an insoluble substance, but rendered soluble by the alkalies which it contains; for if the white of egg, or serum of blood, be dissolved in a large quantity of pure water, and the solution be exactly neutralized by acetic acid, a flocculent precipitate is obtained which is insoluble in pure water, but easily soluble when the latter contains a small quantity of caustic alkali. So obtained by precipitation, it has neither color, odor, nor taste. Albumen contains in one hundred parts

Common dried albumen, not obtained by precipitation, contains, in addition to common salt, phosphate of soda, and carbonate of soda. It can easily be shown that white of egg contains sulphur, by boiling it in a solution of caustic potassa and acetate of lead, when a black precipitate of sulphide of lead will be formed. The photographic student will also observe that albumen contains the elements of ammonia, which is generated during the putrefactive decomposition of this material. The salts which it forms with metallic oxides are denominated albuminates; and the albuminate of silver, which is formed at the same time with the chloride of this metal in the albumen film, is instrumental in producing the difference that exists between a plain print and an albumen print.

Gelatine.

This substance, if it exist in nature, has never yet been obtained otherwise than by the use of boiling water; it is supposed, therefore, by some to be a product of 'the decomposition of albumen or fibrine. All membranes, such as the skin, tendons, cartilage, hoofs, and bones, yield, when boiled at a high temperature, a solution which, on cooling, concretes into a semi-transparent tremulous mass. This substance is gelatine or its congener chondrin, (from cartilage.) The jelly obtained from boiling calves' feet, common size, isinglass, and common glue are familiar examples of gelatine. Isinglass (the dried swimming bladder of the sturgeon) dissolves in water, and yields a very pure form of gelatine. When pure and dry, gelatine is colorless and transparent; it swells and softens in cold water, in which it is very sparingly soluble; but in hot water it dissolves very easily. Alcohol and ether do not dissolve it; it is precipitated by alcohol from an aqueous solution. When dry it can be preserved for an indefinite time without alteration, but in a moist state it undergoes decomposition, becomes acid, and ceases to gelatinize. Long-continued boiling produces the same effect. Some metallic salts produce a flocculent precipitate in solution of gelatine, so does chlorine; but its most characteristic property is that of being precipitated from a very dilute solution by means of tannic acid, the only acid by which it is precipitated. Acting on this principle, skins are converted into leather by the process called tanning; but skins are not boiled in this process, and hence it is supposed that gelatine, after all, is a natural product.

When gelatine is digested in strong sulphuric acid, or in caustic potassa the wine decomposition is effected. Ammonia is invariably one of the products, and among other products we may count sugar of gelatine or glycocine and leucine.

Dry gelatine is found to contain in one hundred parts

Amylaceous or Non-Azotized Substances.

Starch, arrow-root, cellulose, gum-arabic, etc., belong to this class of bodies. They are found in the vegetable kingdom in a free state, and produce by slight changes in the vegetable organization, a great variety of substances, containing no nitrogen, and differing essentially only in the different number of equivalents of water with which they are combined, or, as far as regards chemical equivalents, sometimes not differing at all; for starch, dextrin, arrow-root, gum-tragacanth, cellulose, amidin, all contain the same number of equivalents of carbon, hydrogen, and oxygen, and are all resolved into saccharine substances by treatment with acids.

Starch.

Seeds, roots, tubers, and stems of most plants contain this substance in the form of very minute insoluble granules. If pumpkins, potatoes, or horse-chestnuts be rasped, and the pulp be then well washed on a fine sieve, these granules will pass through the meshes, whilst the cellular tissues will be retained on the sieve. The powder will finally subside, and the fluid above it can be poured off. This substance is starch, which has to be washed several times, in order to get rid of impurities, and especially the bitter principle peculiar to certain seeds and plants. After the white residue has thus been thoroughly purified, it is dried at a gentle heat, by which it concretes and cracks into the form in which it generally exists in commerce. Starch is not only insoluble in water, but also in alcohol. When examined in the microscope, these granules, of an oblong shape generally, exhibit concentric rings by which the starch granule is easily designated from other powders, and frequently the granule of one plant can be distinguished from that of another, as, for instance, that of the potato from that of arrow-root. The latter substance is the starch obtained from the roots of the maranta arundinacea, growing in the West-Indies. The size of the granule varies from 1/600 to 1/250 parts of an inch in diameter. Each granule is regarded as a cell of concrete and insoluble material, holding within a soluble pulp. When boiled, the cells are burst or broken up, and the soluble part mixes with the water and forms a thick gelatinous mass, called amidine. If the solution of starch be dried at a gentle heat and then digested in cold water, the fluid portion can be separated from the insoluble husks or cells, in a colorless, transparent form. A thin solution of starch is precipitated by several bases, as lime, baryta, and protoxide of lead; a large addition of alcohol has the same effect. Infusion of galls causes a yellow precipitate which dissolves when the solution is heated. The best test of the presence of starch is free iodine, which produces a beautiful violet-blue color or precipitate in solution of this substance. The blue color disappears on the application of heat, and returns as. the solution cools.

The substance called British gum is simply starch that has been heated above 240°, when the latter softens and becomes brown and soluble in cold water. If a solution of starch be boiled with a small quantity of dilute sulphuric, hydrochloric, or, in fact, almost any acid, it soon becomes thin and is then called dextrine. The sulphuric acid is afterward removed by adding chalk to saturation, and then by filtering and evaporating the filtrate to dryness. The substance thus obtained resembles gum and is soluble in cold water. By continuing the action of sulphuric acid and the boiling, dextrine is converted into grape-sugar. This conversion is produced also in the act of germination of seeds as in malting.

Gum-Arabic.

This substance is the spontaneous exudation from the bark of the acacia vera and the acacia arabica. In its purest and finest condition, it is in the form of white or slightly yellowish concretions, which are soluble in cold water, forming thus a viscid, adhesive solution. The pure gummy principle, called arabine, is precipitated by alcohol and by basic acetate of lead.

Chloride of Gold.

Gold does not dissolve directly in hydrochloric acid, but it enters into combination very vigorously with moist chlorine, or with chlorine in the nascent state. The menstruum in which it dissolves is nitro-hydrochloric acid.

Gold.--Symbol, Au. Combining Proportion, 197. Specific Gravity, 19.3.
Protoxide of Gold.--Symbol, Au O³. Combining Proportion, 205.
Teroxide of Gold.--Symbol, Au O³. Combining Proportion, 221.
Terchloride of Gold.--Symbol, Au Cl₃. Combining Proportion, 303.

Gold dissolves in a mixture of one part nitric acid and four parts hydrochloric acid. In this mixture the nitric acid becomes decomposed, parting with oxygen, which then decomposes the hydrochloric acid and combines with its hydrogen to form water, whilst the chlorine in the nascent state combines with the gold in the solution. This is afterward evaporated on a water-bath in order to drive off all excess of acid. In this way we obtain a red-brown, deliquescent crystalline mass of the terchloride. If the heat be too great, the salt is decomposed, chlorine is set at liberty, and a protochloride or metallic gold is left, according to the temperature. The terchloride is very soluble in water, ether, and alcohol. The solution has a yellow color and an acid reaction; it stains the skin purple. Ether separates this salt from an aqueous solution very effectually by agitation; and the mixture ascends and forms a layer on the surface of the water, which can easily be separated by decantation, by a syringe, or by allowing the water solution to flow off from a funnel; after which the ether is expelled and collected by distillation.

Most of the deoxidizing agents reduce terchloride of gold, such as hydrogen, carbon, carbonic acid, deutoxide of nitrogen, sulphurous acid, phosphorous acid, and their salts, terchloride of antimony, the proto-salts o f iron, many of the metals, most organic substances, and oxalic acid.

The crystallized terchloride has a, dark reddish-brown color; but if it contains excess of hydrochloric acid, it has a bright yellow color; the solutions partake of the same color; the color, therefore, is a criterion of the purity of this salt. A strong solution of the salt has a dark olive-green tinge, which becomes yellow by dilution. This salt combines with the analogous potassium, sodium, and ammonium salts, giving rise to definite compounds of these double salts, which are very frequently sold in commerce for the true terchloride. The formulas for these three salts are

Aurochloride of Potassium.--K Cl. Au Cl₃ + 5 Aq.
Aurochloride of Sodium.--NaCl. Au C1₃+4 Aq
Aurochloride of Ammonium.--NH₄ Cl. Au C1₃ + 2 Aq.

All these salts, as well as the double salt of gold and calcium, are used in toning. They are formed by neutralizing the hydrochloric acid in excess in the terchloride by means of the respective carbonates of the preceding metals.

Refuse gold solutions are reduced in general by either sulphate of the protoxide of iron or by oxalic acid. The brown powder which subsides is well wished, first with water, then with boiling hydrochloric acid; this is pure gold in a fine pulverulent form, which can be used for gilding and enameling, or for making pure terchloride.

The gold coins of the country are alloyed with either silver or copper, which can be separated by various methods. Both the silver and copper may be removed at the same time by the following means: melt, for instance, a gold dollar together with ten times its weight of silver (ten five-cent pieces) in a crucible; when melted, pour it out on a clean stone, and afterward pass the lump between a pair of rollers so as to reduce it to very thin foil. Digest the foil in pure nitric acid, which will dissolve the copper and the silver, and leave a residue of a bright cinnamon color. Wash this residue, which is gold in a very porous or pulverulent condition, and then dissolve it, as before directed, in vitro-hydrochloric acid; evaporate to dryness, dissolve, and rectify by ether.

Whenever silver is alloyed with gold, it is precipitated during the solution in aqua regia as the insoluble chloride, which can be removed by decantation of the chloride of gold. The copper is afterward precipitated as the green carbonate by adding carbonate of soda to the solution as long as effervescence is produced, which is separated, in like manner, by decantation.

If steel be dipped in an ethereal solution of the terchloride of gold, it becomes covered with a film of reduced gold. Dry gilding is performed by coating the article with an amalgam of gold, submitting the same to heat, so as to drive off the mercury, and then burnishing the gilded surface. An amalgam of gold consists of a solution of gold foil to saturation. The article is first dipped in a solution of nitrate of mercury, and then covered with amalgam.

The gold solution for electro-gilding is made by dissolving to saturation the terchloride of gold in a saturated solution of cyanide of potassium; this solution can afterward be diluted ad libitum.

Nitrate of Uranium.

Uranium is a metal which is not very abundant; in combination it occurs in the mineral pitch blende, as the black oxide; with silica, oxide of lead and oxide of iron, as uranmica or chalcolite, and as uranite in combination with lime and phosphorus.

Uranium.--Symbol, U. Combining Proportion, 60.
Sesquioxide of Uranium.--Symbol, U₂ O₃. Combining Proportion, 144.
Nitrate of the Sesquioxide of Uranium.--Symbol, U² O₃, NO⁵. Combining Proportion, 198.

This salt is obtained directly from pitch blende by treatment with nitric acid. The ore is first pulverized and acted upon by nitric acid; and the solution is then evaporated to dryness. The residue is then washed with water, which dissolves the nitrate and leaves a quantity of sulphate and arseniate of the sesquioxide of iron. The liquid still contains salts of copper, lead, and arsenic; these are removed by passing a current of hydrosulphuric acid through the solution, which precipitates all these metals. The solution decanted or filtered from the sulphides of the above metals is evaporated to dryness, and the residue is again treated with water, which takes up the nitrate and leaves a residue of sesquioxide of iron. The solution is now evaporated and crystallized.

Nitrate of uranium is a yellow salt, which is very soluble; it contains six equivalents of water, which by heat can be expelled, and by greater heat the salt is decomposed. The alkaline carbonates all produce yellow precipitates with the salts of the sesquioxide; whilst ferrocyanide of potassium produces a red-brown precipitate. This salt has been latterly used in the toning-bath along with the terchloride of gold.

Acetate of Soda-Citrate of Soda,-Phosphate of Soda.

These three salts are easily prepared by adding to each of the acids, acetic, citric, and„ phosphoric, carbonate of soda as long as there is any effervescence. The solutions are then evaporated and crystallized.

Acetate of Soda.--Symbol, Na O, C₄ H₃ O₃+6 HO.
Citrate of Soda.--Symbol, 3 Na O, C₁₂. H₅ 0₁₁.
Phosphate of Soda.--Symbol, 2 Na O, HO. PO₅.

Carbonate of Soda.

Symbol, Na O, CO₂.

This salt is now obtained from chloride of sodium or common salt. The latter salt is first decomposed into sulphate of soda; the sulphate of soda is next roasted with charcoal, by which it is converted into sulphide of sodium,; and finally the latter substance, by roasting with powdered limestone and coal, is reduced to carbonate of soda.

Carbonate of Lime.

Symbol, Ca O, CO₂. Combining Proportion, 50.

This substance occurs in great abundance, as chalk, marl, marble, and limestone. Chalk is sufficiently pure for the purpose alluded to. When added to the terchloride of gold, carbonic acid is liberated, and chloride of calcium formed, giving rise to the double salt, aurochloride of calcium, which is to be decanted from the insoluble residue. This salt is more easily prepared in a definite condition than any of the preceding aurochlorides; and on this account its employment in the toning-bath is more reliable and to be recommended.

Chloride of Ammonium.--Symbol, NH₄ Cl. Combining Proportion, 52.
Chloride of Sodium.--Symbol, Na Cl. Combining Proportion, 58.
Chloride of Potassium.--Symbol, K Cl. Combining Proportion, 74.
Chloride of Barium.--Symbol, Ba Cl. Combining Proportion, 104.
Chloride of Calcium.--Symbol, Ca Cl. Combining Proportion, 63.

All these chlorides can be so easily prepared by saturating hydrochloric acid with their respective carbonates as long as effervescence is produced, that it is not necessary to describe them separately. There is this to be remarked about them in their application to photography, that the same quantity of either (a thing which I need scarcely remark) will not produce the same effect. Of those already mentioned, the chloride of ammonium by weight requires to be used in the smallest quantity, whilst the chloride of barium, when just twice as heavy, is only equally efficacious in producing a given quantity of chloride of silver.

The iodides and bromides, as also gallic acid, have been already described. We shall, therefore, proceed to the minutiae of the manipulation of positive printing by contact.