This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1913" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1913.
There are several advantages in developing with Pyro, that many photographers have lost sight of, since developing papers have come to be more generally used, and in getting away from Pyro the printing quality of the negative has almost invariably suffered.
At the start developing papers were more harsh than they are to-day, and the photographer who began their use several years ago, found it necessary to make softer lightings and flat negatives to get the best results.
This was easy with a modification of the same coal-tar developers used for the papers, the negatives having practically no color, less gradation and being much thinner than they appeared to be.
But about the time coal-tar developers came to be more generally used for developing plates, the quality of developing papers began to improve. They came to have a longer scale of gradation, would better reproduce the quality of a good negative, gave results nearer to Aristo in quality and no longer required the flat, lifeless negatives.
The grey, colorless negative being no longer strong enough for the best developing papers, many photographers began to develop their negatives stronger. Here is where many a man's negative quality has gone down hill and hasn't come up again. Just here we will look into the action of Pyro as a developing agent and see what its advantages really are.
First of all, when you make a certain lighting, you expect to reproduce it in the negative and print just as you see it on the ground glass. Develop the plate with Pyro, balancing your sodas to meet local conditions, and you can get this result, and at the same time have a negative with any degree of printing density.
With coal-tar developers the grey, colorless negatives are thinner than they look and must be developed beyond the point where they seem to have a full scale of gradation. With longer development for greater printing density, the highlights begin to block up and the gradation of the negative is cut down step by step until the final negative and print from the same are not at all what you saw in your lighting.
Make the lighting soft and flat to reduce the contrast in the negative, and still your picture will not be round, because you have reduced the scale of gradation and you can't get it in the print if it isn't in the negative.
The coal-tar developer can not give the printing color you get with Pyro, and you could not even stain such negatives and get the same result, for the Pyro stain is not even over the entire negative. It is greatest where there is the greatest silver deposit, so the Pyro negative is really intensified - has more brilliancy and gradation than it appears to have.
It requires shorter development to get a good Pyro negative, and in this there is another advantage. The longer development with coal-tar developers to get density, also develops up any halation exposure, which, if present, is on the under side of the gelatine and is not reached by the shorter development with Pyro.
The only advantage with coal-tar developers for plates is that the negative looks good, but the customer doesn't buy the negative.