The international photographer (Jan-Dec 1939)

,i< rooms by air currents, because of the oxidation stain it forms on aiiliing it comes in contact with. Pyro, being staining in its action, is n1 entirely for negative developing. ELON, METOL, and PICTOL ( Monomethyl-para-aminophenol sul plit) which are identical in composition so far as results are concerned a i are made, in common with many other developing agents, principally n coal tar. It is a white flaky powder insoluble in alcohol. Elon is u I for deveolping both negatives and prints. ; HYDROQUINONE, made from benzine, which is first converted into aJline and then oxidized. It occurs in small needle-like white crystals. T|s agent is seldom used alone, being most generally found in comb^ition with Elon. It may be used for both negatives and prints. I These agents just mentioned above are the ones most generally used. Rivever, since the advent of the miniature camera and the necessity for li ^rain developing, such developing agents as para-phenylene-diamine ili the base and hydrochloride forms I and glycine are used to a conirable extent. Each developing agent differs in its action and a difIVnl formula is necessary for the use of each. (See Chapter 5.) ACCELERATORS: As has ben stated, reducing agents must usually b in an alkaline solution so that development can take place. The dies used for this purpose are the carbonates and hydroxides of sodium potassium, usually the former. The alkaline solution serves a double pose, it not only renders development possible at a rapid rate, but :ens and swells the gelatine emulsion, permitting more rapid access ;he reducing agent to the entire thickness of the emulsion. The various elerating agents are given as follows: SODIUM CARBONATE: This is by far the most popular accelera It is marketed in three forms: Crystals containing ten parts of :er of crystallization and 30% by weight of the carbonate; crystals e taining one part of water and 85% of carbonate, and the dry, or Hydrous powder containing 98% carbonate. ' POTASSIUM CARBONATE: This alkali, which is also known as Hs of Tartar, is sometimes used instead of sodium carbonate. It is rilre soluble than the former and a stronger alkali. This chemical is 1 pared in the same manner as sodium carbonate, being obtained comrjrcially as a dry powder. It has the disadvantage of being very deIjuescent and must be kept in well sealed bottles. ! SODIUM HYDROXIDE: This alkali, which is also known chemically < austic Soda, is occasionally used in developing solutions requiring strong alkali. Caustic Soda is a very strong alkali with a decided corilive action. It is marketed in white brittle sticks, and in little pellets, lis hygroscopic and upon exposure to air absorbs carbon dioxide and 1'ter. Therefore, it should be kept in well stoppered bottles. j POTASSIUM HYDROXIDE: This alkali, sometimes substituted for (■ustic Soda, is very similar to it and prepared in the same manner, fe) [ Obviously compounds which contain a large amount of water of ( stallization must be used in greater weights to obtain the same strength !en by the powder form. Likewise those which are Efflorescent, Delitescent or Hygroscopic are of uncertain strength after having been ex]'sed to the air for any great length of time. The powder form is :ferable in almost all cases where chemicals can be obtained in either m. An exception being sodium carbonate, which is more stable when tained in the monohydrated form. In compounding developers it is sometimes necessary or desirable to bstitute one alkali for another. Since they are of different strengths, following table with Anhydrous Sodium Carbonate as unity indicates if; weight of each to be used to produce the same results: ! Sodium Carbonate (Anhydrous) (slightly Hygroscopic) . . 1.00 1 Sodium Carbonate (Monohydrated) (very stable) .... 1.11 Sodium Carbonate (Crystals) (efflorescent I 2.69 Potassium Carbonate (Anhydrous) (slightly Hygroscopic! . 1.30 Potassium Carbonate (Crystals) (efflorescent) 1.63 Sodium Hydroxide (Caustic Soda) (deliquescent I 75 Potassium Hydroxide (Caustic Potash) (deliquescent) . . 1.05 Ammonia (Specific Gravity of .808) 32 PRESERVATIVES: It has been stated that the various reducing ents used in developing solutions are readily oxidized by free oxygen ,d to prevent this occurrence we add to the solution a substance which s the power of consuming all the free oxygen present. This is readily ien in the case of pyro, which oxidizes very rapidly when little or no ■ eservative is present. Negatives developed with pyro under these contions present a heavy brown stain, but by adding sufficient preservative, negative with very little stain can be obtained. The compounds used r preservatives are of the sulphite group of sodium and potassium, ually the former. Sodium sulphite, the chemical most generally employed, is prepared ' blowing sulphur-dioxide gas through a solution of carbonate of soda. ie resulting solution is cooled and the sulphite crystallized out of it. "hese crystals contain seven parts of water of crystallization, and about 1% of the sulphite. When exposed to the air, these crystals effloresce, give off water, forming a white powder on the surface. Also, upon posure to air, sulphite is oxidized into sulphate, which is useless as a •eservative. Therefore, sodium sulphite should be kept in well sealed /ntainers. By drying the crystals, desiccated sodium sulphite is obtained. This mtains about 95% pure sodium sulphite. By precipitating the sodium dphite from hot solutions, a powder which contains about 98% sodium sulphite, or Anhydrous sodium sulphite is obtained. This is the form that it most commonly used. RESTRAINERS: There are a number of chemical compounds which have the power of slowing down the action of an alkaline developer, but of all these, only one is in common use. This one is Potassium Bromide. All soluble bromides, chlorides and iodides can act as restrainers, and to a less extent so can some of the alkaline citrates. However, the only two restraining agents which need be mentioned in this text are as follows: POTASSIUM BROMIDE and POTASSIUM IODIDE: Potassium Bromide is universally used as a restrainer in developing solutions, while Potassium Iodide is occasionally used when making prints to reduce "abrasion Marks" and "fog." (371 FIXING AGENTS THE FIXING AGENT: Although there are numerous chemicals now known which will remove from the developed image the unacted-upon grains of silver bromide, there is but one in common use. THIOUSLPHATE OF SODA (Hypo): This universal fixing agent can be made by boiling together sodium sulphite and sulphur. Commercially it is made by treating calcium thiosulphate with sodium sulphate. It is obtained commercially in large crystals containing seven parts of water of crystallization. It is usually pure, although sometimes, contaminated with foreign matter, due to careless handling. Hypo is somewhat hygroscopic, and should be stored in a dry place due to this property. ACID USED IN FIXING BATH: To neutralize the akali carried over into the fixing bath, a fairly large amount of relatively weak acid is necessary, rather than a small amount of strong acid. The amount of alkali which an acid can neutralize depends upon the total amount of hydrogen present, and not upon the disassociated portions only. The acids strongest in acidity are the so-called "mineral" acids such as sulphuric and nitric, while the weakest are the "organic" acids such a> citric and acetic. The latter kind, however, have the power of neutralizing large amounts of alkali. ACETIC ACID is most commonly used in fixing baths. In dilute and impure form, this acid is known as vinegar and is prepared from the fermentation of apple juice, grain, etc. The strongest form of acetic acid is known as GLACIAL acetic acid, which is 99% pure. Glacial acetic acid may be diluted with distilled water to make acetic acid of any required strength such as 28%, which is called for in nearly all photographic formulas using this acid. Acetic acid is a colorless liquid with a very pungent and sometimes irritating odor. In concentrated form, it has a decided corrosive action. As a substitute for acetic acid, some fixing formulas call for the use of SODIUM BISULPHITE, which supplies the necessary acidity, although it does not furnish the reserve of neutralizing power that acetic acid does. CHEMICALS USED IN HARDENING: In order to prevent undue swelling and softening of the gelatine emulsion in subsequent washing, certain chemicals are added to the fixing bath or are used at other times in the various photographic processes. These chemicals which have a tanning or hardening action upon gelatine, are mainly certain alums, there being only one other chemical (Formalin). ALUM is a compound of sodium potassium or ammonium with aluminum. If the hydrogen of sulphuric acid is replaced by aluminum, we get aluminum sulphate. This, in combination with the substances mentioned above, i. e., sulphates of sodium, potassium or ammonium, forms the compound used in photography. POTASSIUM ALUM is most commonly used as the hardening agent in acid hardening fixing baths. It is obtained commercially in the form of clear crystals or a white powder. These forms are quite stable. The powder form, however, is somewhat subject to lumping upon exposure to the air. POTASSIUM CHROME ALUM is a compound of sulphate of potassium and chromium. Potassium Chrome Alum is often used in place of the ordinary potassium alum. It is obtained commercially in a very pure state in the form of violet crystals, which are red by transmitted light. These, when dissolved in water, form a violet solution, which upon heating, changes to a green due to a chemical change which is injurious (38) to its hardening powers. Potassium Chrome Alum has a greater hardening power than potassium alum, which is further increased when the solution is made slightly alkaline. This hardener, used for negatives only, is often used in warm weather as a separate or supplementary hardener in addition to its use in the regular fixing bath. FORMALIN is a solution of formaldehyde in water, the commercial solutions containing about 40% formaldehyde. Formalin has the property of hardening gelatine more than any other hardener and for this purpose is used in weak solutions — not over 5%. In less than a minute such a solution will render gelatine completely insoluble in boiling water. Formalin must be used in a neutral or alkaline solution, as in the acid state it loses its hardening power. Care must be taken to prevent overhardening when using this chemical as it is possible to carry the hardening to a point where the gelatine film will become brittle and crack. Formalin gives off a strong and unpleasant odor, very irritating to the mucous membrane. VTERNATIONAL Photographer for August, 1939