Re: casting with hide glue - shrinkage
What are you really trying to do? Is this a cast - a positive form intended for permanent display - or a mold? You mention you plan to "invest" it - do you want to burn it out and replace it with metal?
As it's traditionally used, it's a mold material, not a casting material. It was the earliest flexible mold material, primarily used for creating ornamental plasterwork. It has mostly been superseded by modern silicone and urethane rubber compounds, which are easier to use and last longer. You should apply it to the thing you're molding as soon as it's melted (in a double boiler), let it cool, and cast right away. It won't last long - as you've noticed, it tends to shrink, and when it's dried out it won't release or flex well.