I would suggest that you take some other photographs printed on similar paper, and experiment. Sweat onto them, let it dry, then try different techniques.

A: Put a puddle of distilled water on a stain, swirl with a Qtip for N minutes, then pull the water off initially with a paper towel, and finish with a piece of lens tissue. (The water wicking into the paper towel won't shed onto the print.

B: Same idea, use distilled water mixed 200:1 with photoflo.

C: Start with 1 liter of warm water and 2 drops of uncoloured dish detergent. Swirl, blot, and immediately rinse with B, above. The idea is that detergent will remove residual skin oils.

D: If any of the above worked, try repeating it. If it worked slowly,wet it repeatedly, then dry it by putting down a piece of lens tissue backed up with a paper towel. (A towel by itself will leave towel lint embedded in the gelatine.)

E: is the stain lighter than the print? If so you may be able to use retouching colours to fix it. Get lots of practice first. It's an acquired art.

Examine with a 10 power jewelers loup. You are looking for the following:

• is there an actual change of colour between inside the ring and outside the ring?

• is all the discolouration just in the ring itself? (Surface tension favours junk collecting at the point where the liquid meets the solid surface. Coffee stains are rings, not spots.

• is there a height difference between inside and outside? (The gelatine has swollen and not shrunk back. You may want to use a very low angle illumination to check this. An LED flashlight laying on the table works well.

• is there stuff stuck to the print?

After working on it, do you still have a stain? Have you moved the stain? If it's a ring stain, not a spot stain, can you move the edge of the stain to an inconspicuous location?