The question is often asked: How can one determine the strength of a fixing bath for paper and when it is exhausted?

For all practical purposes, the number of prints that can safely be fixed in a given amount of fixing bath of a certain strength is enough to know, for a fixing bath should never be over-worked. But that is not an answer to the question.

The rule is to use 32 ounces of an acid fixing bath in which there is one ounce of hypo to each four ounces of water, for one gross of cabinet prints or their equivalent.

If this rule has not been followed, a fixing bath can be tested with some little trouble and its strength determined with a fair degree of accuracy.Cut a number of one inch test strips of the unexposed paper for which the fixing bath is to be used. Immerse these in the bath for varying lengths of time; that is, fix one strip 30 seconds, another one minute, another two minutes, etc. Note the time of fixing on each strip, remove them from the fixing bath at the end of the given time and wash thoroughly. Then immerse the strips in a 1% solution of sodium sulphide.

The presence of very minute quantities of unfixed silver in the emulsion of these strips will cause a brown or yellowish brown stain to appear when the strip is placed in the sulphide solution.

In this way the rate or speed of fixation of a fixing bath may be determined, for a strip treated in this way is completely fixed if it does not discolor in the sulphide solution.

The result of this test does not indicate, however, that a large print will fix in the same length of time as a small test strip. It is impossible to insure complete fixation over a large surface in the same time indicated by a small test strip and when a number of prints are being fixed the time will be still slower.The best way of arriving at a margin of safety for a fixing bath is to make a test as outlined above for a fresh bath. If a partly used bath is then tested and found to require twice as long to fix, it is sufficiently exhausted to be discarded as unsafe for use.

There is also a question which arises at times as to the rate of fixation and the strength of the bath that will give the greatest fixing efficiency.It should be understood that this has always had the careful consideration of those who are responsible for the fixing formulas and with very rare exception these always call for a 25%, or one to four solution of hypo.

Testing Fixing Baths And The Rate Of Fixation StudioLightMagazine1923 63

PORTRAIT FILM NEGATIVE, VITAVA PRINT

By Fernand de Gueldre Chicago, III.

The idea that the more hypo you put into a fixing bath the faster it will fix is an erroneous one. The rate of fixation increases up to a concentration of about 40%, beyond which it decreases, until with a concentration of 80% or 90% a film or plate will refuse to fix at all.

As a result of experiment it has been found that a 25% or, at the outside, a 30% solution is the most economical. That is, it is better to use a bath of this strength and throw it away when it has become stained and is slow in its fixing action, rather than to attempt to use a 40% solution until it becomes exhausted. Two 20% fixing baths would do their work more efficiently and correctly fix a greater number of negatives than would be possible with one 40% bath. But the 25% solution usually recommended has been found by test and by experience to be most efficient, so don't waste good hypo by trying to make it stronger.

If there is any special reason for fixing a few negatives very quickly without regard to expense, then the 40% solution will be found to give the maximum of speed. For general work, however, and for the greatest efficiency and economy, stick to the 25% solution recommended by the manufacturers. Use it until reasonably exhausted and then discard it for a fresh bath.