MITRA Forum Question Details

​Animal glue typically has an odor, but some manufacturers, in partucular manufacturers in the United States, add a deodorizing compound along with other additives to reduce or eliminate odor. If you have dried animal glue and it has an odor it may not be putrified. However, if you have a solution of animal glue and it has visible discoloration and odor, it is likely putriified.

I've never seen rabbit skin glue dry granules spoil in storage (clump, yes, but not spoil. When dry RSG is stored for a long time, however, it can become less soluble. When old glue is prepared, undissolved granules may settle out, resulting in a weaker sizing. ​Rabbit skin glue is pretty affordable, and in my opinion it's worth obtaining fresh stock. If you prepare a batch and it fails to gel on a cold spoon, or a significant amount remains undissolved, I think it's best to just get a new bag. 

I would add that glue made from genuine rabbit and hare tends to be slightly more opaque and have a stronger odor (not putrid but a noticeable smell) that the materials packaged as RSG but that are likely a bovine hide glue. The hide glue may be more refined because it is sourced from producers who also sell to the food industry (this is only a supposition).

BTW, I am not trying to proclaim the superiority of glue made from rabbit skins. It is all collagen. Different sources and processes will create glues with greater or lesser bloom strength, and are more or less appropriate for certain purposes. Actually, there is no mention of the use of rabbit skin glue in painting manuals before the 19th century. Pig skin (from glove clippings) and parchment scraps were the norm in the early manuals.