Don’t make me go Medieval.
...Its supreme advantages are very, very fast bonding; and easy debonding with hot water if an instrument must be disassembled for internal repairs. [snip]
Because the glue is hygroscopic, it continually absorbs moisture from the atmosphere, causing the glue to swell and shrink. Over time, this constant flexing causes the brittle oil paint to crack. Modern substitutes for rabbit skin glue are available, such as Gamblin’s PVA size [2] and Golden Acrylics’ GAC100. These substitutes do not have the hygroscopic properties of rabbit skin glue, while still being very slightly hygroscopic, and should not cause the damage to oil paints that rabbit skin glue does. However, these modern replacements do not stiffen and tighten the canvas as well as rabbit skin glue does, so some artists still prefer to use rabbit skin glue.
Since they hadn't invented gunpowder yet, the cry was, "keep your armor dry!"
That really is a fascinating article. I happen to know a certain woodworker that’ll go all geeky over it.