Hi ! I work at an Universit in Sao Paulo State, Brazil and wold like to know how can I get positive images in B&W from negatives films . I have made some tests, and the results are well, but in this process I have to over espose the film 3 or 4 spots. Thank's for any answer. Milito

-- Alexandre Milito Neto (milito@francanet.com.br), April 07, 1999

Answers

If you are asking how to get a black and white print from a negative, the answer is to print it. However I think you are trying to ask how to get a transparency from a black and white negative film. The answer to that question is to reverse process it.

The quickest way to reverse process a black and white negative film is to purchase one of the comercial kits for doing just that. There are two kits that I know of. One is the T-max reversal kit from Kodak. The other is the reverse processing kit from Photographers Formulary. They both include directions for their use. The Photographers Formulary kit is based on the chemistry in an article by Hans Deitrich in Darkroom and Creative Camera Techniques from March/April of 1988.

If you want to mix all the bits together you can do that too. Deitrich's article used to be on the web but that site got shut down for copyright violations. There are a number of other articles that are floating around that also describe the chemistry.

To go into detail on the reagents and techniques would be a bit much for this response but I will give an overview of the process here. First you develope the negative image on the film. This is done using a modified developer with added potasium bromide some sodium carbonate, and a small amount of sodium thiosulfate (AKA hypo). This negative image is then bleached using either potasium dichromate or potasium permanganate. The bleach is then cleared from the film and the remaining silver is exposed either by direct exposure to light or chemically. The film is developed a second time to develope the positive image, then fixed, washed and dried as per normal.

One word of caution, black and white film bases tend to melt in the heat of a projector so make sure you have a relatively cool projector or you will watch it all melt before your eyes.

If you dont want to use one of the kits and can't find the recipes for the reagents on the web or elsewhere, e-mail me and I can send you the necessary protocols. And yes, you do have to under rate your film, the exact amount varies with the film you use and the desired outcome. Good luck.

-- Fritz M. Brown (brownf{DHWTOWERS/TOWERS3/brownf}@dhw.state.id.us), April 08, 1999.