why do pictures curl up when there is hot weather?

Factual Questions
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Chief’s Domain - http://www.seas.ucla.edu/~ravi

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In general, things curl up because they have two sides and one of those sides changed length with either temperature, or humidity. In the case of pictures, it is often the humidity, which causes the paper side to expand, and the chemical (shiny) side to remain more stable. That makes it curl up.

<p align =“center”>Tris</p>

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Quote:
“In general, things curl up because they have two sides and one of those sides changed length with either temperature, or humidity. In the case of pictures, it is often the humidity, which causes the paper side to expand, and the chemical (shiny) side to remain more stable. That makes it curl up.”
You are basically right, but you are wrong on one point. Most photographic papers today are RC papers (resin coated). Water hardly penetrates the actual paper, if at all. The photo side is covered by an emusion which does swell and shrink with humidity. In the old days of photography, there were no RC papers, and the emulsion was put directly on the paper; which was either SW (single weight) or DW (double weight).

The advantage of having RC paper is that most of them have the developer in the paper itself, and after being put in the developer, the paper can be taken out and it will still develop. But the main advantage is that with the paper not soaking up the chemicals, it is alot easier to wash out the chemicals from the print and dry them. Without a chemical called hypo-clear, which helps to remove the fixer from the print, fiber-based papers should be washed in continuously flowing water for at least an hour. By using hypo-clear, you can cut that down to a 1 minute water wash, followed by a 1 minute hypo-clear wash, followed by a 1 minute water wash. With RC papers, all that is needed is a 4 minute water wash with no chemicals.

Also, you need a paper dryer to get decent looking results from fiber-based papers. With RC papers, you can prop them up for 15 minutes or so and they are perfectly dry.

The only drawback is that RC papers are not as durable as fiber-based papers, as the resin does start to come off the paper after time…