https://www.rigb.org/education/loreal-young-scientist-centre/teaching-resources1/pinhole-cameras

“Make your own developing solution!

You can use dried mint, coffee, or basil – anything with caffic acid, which converts the colorless silver ions in the photographic paper into dark silver to produce a negative image.

If using mint, make a strong mint tea by stirring 10g of dried mint leaves into 200ml of hot water. Leave to brew for 15min, then strain through a coffee filter into a new container. In another glass, measure out 200ml of cold water and add two 1000mg vitamin C tablets. Then gradually add 10g of bicarbonate of soda while stirring to help break down the bubbles. The bicarbonate helps the vitamin C dissolve and creates the alkaline conditions needed for the developer to work. Mix the two solutions together and leave to rest.

Make an acidic stop solution to halt the chemical reaction started by the developer. Just mix 5 ml of lemon juice with 200 ml of water.”