Any tank continually used for developing will collect a corrosive substance which cannot be got rid of by ordinary washing. If allowed to remain this affects the developer, and the negatives are apt to be fogged, or stained and streaked around the edges. There need be no trouble of this kind with a tank if it is thoroughly cleaned at least once a month. The process is so simple that there is no excuse for a dirty tank. All you have to do is to make a solution of equal parts acetic acid and water, pour it into the tank, fasten the lid and shake well. This will clean the tank in a few minutes.

Causes Of Weak Negatives

Photographers sometimes get a batch of weak negatives, and very often are unable to discover the reason. It may arise from any one of three causes. Two of them - under-exposure on account of poor light and underdevelopment on account of the developer being too cold - are unlikely at this time of the year. The third cause is one which is to be found summer and winter alike. It is the use of a developer containing chemicals which are below the proper strength.

If a formula has the proper proportions of pyro, sulphite and carbonate, either by weight or hydrometer test, and it gives weak negatives, do not try to get more density by increasing the amount of pyro. The probability is that the developer is lacking in carbonate.

As every professional knows, the function of carbonate in the developer is to act as an accelerator. It opens the pores of the gelatine and helps the reducing agent, pyro, to act more freely on the granules of silver which have been exposed to the light.

Causes Of Weak Negatives StudioLightMagazine1916 150

FROM AN ARTURA IRIS PRINT

By Edward H. Weston Tropico, Cal.

The plate-maker gives you a developer formula which assumes that carbonate of a certain strength will be used. Obviously when you use a carbonate which does not come up to this standard you alter the working of the developer. It is necessary to take into consideration the difference between the various brands of carbonate of soda. Two brands of carbonate might test the same when dissolved in water, but one would have a stronger alkaline reaction than the other. Many brands contain large quantities of bi-carbon-ate which, though helping to raise the hydrometer test, does not have an accelerating action in the developer. The hydrometer only shows the amount of solids in solution - the test has no value unless the nature of the solids is known. For this reason C. K. Co. Tested Carbonate of Soda, which must contain more than a certain high percentage of pure carbonate before it can pass the laboratory test, would not weigh more, nor test higher, than an equal quantity of carbonate containing impurities. The difference in the developing action, however, would be unmistakably shown in the negative.

There is probably no chemical which varies so much in the strength of different brands as carbonate of soda. C. K. Co. Tested Carbonate is uniform in strength and action, and its increasing popularity among professionals is undoubtedly due to this fact. By using C. K. Co. Tested Carbonate you remove the chief cause, at this time of the year, of weak flat negatives.

In nearly every photographic process the strength and purity of the chemicals used are of as great importance as the strength of the carbonate in the developer. You can easily safeguard yourself by always using Kodak Tested Chemicals. They have to stand the most searching analytical tests in the Kodak laboratories because it is of vital importance to the Kodak Company that their plates and papers should be worked to the best advantage by their customers. It was to remove one of the chief causes of unsatisfactory results that the Kodak Company decided not to send out any chemical without a guarantee of its strength and purity. The sign of that guarantee is the C. K. C. Tested Chemical Seal.