Eastman Portrait Film, Artura Print From a Demonstrator's Negative.

Eastman Portrait Film, Artura Print From a Demonstrator's Negative.

Citric Acid is obtained chiefly from lemons, the juice of the lemon being neutralized with chalk or lime, forming calcium citrate, from which the citric acid is prepared by decomposition with sulphuric acid. Citric acid is frequently adulterated and care should be taken that only pure crystals are used; a specially pure product is packed by the Eastman Kodak Company.

When acetic or citric acids cannot be obtained for the fixing bath, the only substitute which appears to be generally available is bisulphite. Bisulphite of soda, NaHSO3, is intermediate between sulphite of soda and sulphurous acid, and is therefore equal in acidity to a mixture of equal proportions of these two substances. It makes a satisfactory acid fixing bath but does not give quite so good a reserve of available acid in the bath as acetic acid does. This is of importance particularly in connection with the hardening agent used in the fixing bath.

The commonest hardening agent is potash alum, the alum having the property of shrinking and tanning gelatine.

Alum is a compound sulphate of sodium, potassium or ammonium and aluminum. If the hydrogen in sulphuric acid be replaced by potassium, we get potassium sulphate, K2SO4, while if it be replaced by aluminum, we get aluminum sulphate, Al2 ( SO4)3. The aluminum sulphate combines with other sulphates to form the alums, of which the commonest are potassium alum and ammonium alum. Sodium alum does not crystallize well, but the potassium and ammonium salts crystallize in large, clean crystals and are convenient in use. Ammonia alum has the disadvantage that if it becomes alkaline, ammonia may be liberated, which, of course, cannot happen with potash alum, but as potassium salts are difficult to obtain, ammonia alum has generally taken the place of potassium alum. Alum is among the substances specially included in the list of Eastman Tested Chemicals, since its purity is of considerable importance for its photographic use.

Chrome Alum, which is often used in the place of ordinary alum, does not contain any aluminum in spite of its name. It is a compound sulphate of potassium sulphate or ammonium sulphate with chromium sulphate, of which the formula is Cr2(SO4)3, the chromium taking the place of the aluminum present in aluminum sulphate. Chrome alum is prepared commercially in large quantities and of a high degree of purity. It occurs in violet crystals soluble in water, its solution in cold water being violet but going green on heating, owing to the change in the composition of the salt. Chrome alum has greater hardening power than ordinary alum and is often used in place of it for fixing baths, its only disadvantage being its greenish color, which makes the fixing bath look somewhat dark. Chrome alum crystals lose water on keeping, thus increasing in strength weight for weight.

When acetic acid cannot be obtained and the fixing bath is made up with bisulphite it is necessary to substitute chrome alum for ordinary alum. The reason for this is that when a solution of aluminum alum containing sulphite loses its acidity, as it may in a fixing bath, due to the carrying over of the developer, basic aluminum sulphite is precipitated, and this makes the solution muddy, so that if a fixing bath is made with ordinary alum and bisulphite, it will show a precipitate after a time which looks like sulphur, but which is really this basic sulphite of aluminum. Chromium does not form the corresponding compound very easily, so that by using chrome alum with bisulphite a fixing bath is obtained which can take the place of the acetic acid fixing bath with good results.

Formalinis a solution of formaldehyde, a gas having a very strong odor. The commercial solution con-tains 40 % of formaldehyde and has the property of hardening gelatine very powerfully, a 5 % solution rendering the gelatine of a film completely insoluble in boiling water in less than a minute. Formalin is, however, somewhat unpleasant to use and there is considerable danger of producing reticulation with it, so that it is only employed when extreme hardening is required and chrome alum does not harden the film sufficiently.

It is important not to overwork a fixing bath, because as the fixing bath becomes saturated with silver the film or paper will carry this silver into the wash water with it and if not properly washed the silver salt will remain in the finished photograph and will decompose into silver sulphide in time, producing stains. A gallon of the standard strength fixing bath will fix a gross of 8x10 prints, and when these have been fixed a fresh bath should be used.

The Chemistry Of Fixation StudioLightMagazine1919 102