I have conducted more tests for the dry tintype to solve the contrast problem and have thus far made these findings.

The addition of up to 0.1gm of citric acid added to each ounce of developer seems to have made an improvement in the contrast of the image. Exposure was 3sec,f:32, sun. Developed for two minutes. 4 or five seconds may have been a little better.

There was also a little more contrast in developing the plate for 90 seconds without the use of citric acid and exposing for 3 seconds. Aparently the longer time in the developer allows fog to set in without acidulation.

I also tried using white vinegar and also got a better contrast although it was added arbitrarily so I do not know exactly what is the best preportion to use, I am guessing about 1ml per oz. of dev or so.

As in collodion work the addition of acids in developer will increas contrast but also require more exposure.

Your results may vary.