We got back from Blacktown Medieval Fair last weekend and one of the early jobs (other than hanging the canvas) was to see what damage had been done to the leather vessels. They always end up with hairline cracks from use, sometimes the seams will weep and sometimes you get a dark, damp patch spreading from one spot on the surface.
The easiest way to fix these is to pop them in the oven at about 60-70° C for three or four hours and then let them cool down in the oven. There’s a pizza tray under the newspaper so the wire oven shelf doesn’t get imprinted in the base of things.
The gyspen in front was resealed, as was the costrel. The taller bottel, however needs to be cleaned out and resealed properly. It was knocked over a couple of times and I did see two people reach over, pick it up and squeeze as hard as they could.
Watch the temperature, leather is a natural polymer, with collagen fibres linked together with the tannin. Putting too much energy in by overheating will drive more polymerisation, resulting in leather that is brittle and can shatter if hit.
John Waterer in Leather in Life Art and Industry (pp35-6) states that jacks and bombards were hung in the smoke from the fire to cure. I reckon this is a slightly more gentle version of the same treatment.
Brown, D. & McMillan, M. M., The Chemistry of the Leather Industry http://nzic.org.nz/ChemProcesses/animal/5C.pdf
Mathias, L., Natural Polymers http://www.pslc.ws/macrog/natupoly.htm