Mold will grow on almost any organic material that is warm, wet and
not loaded with antifungal agents.  Mold is really tough stuff and
anything that prevents its growth it is not something I would like
to have around with a couple of exceptions.  One is oxygen which is
conveniently supplied as hydrogen peroxide.

A little bit of mercury will stop mold but a little bit of mercury
is too much for me.  They used to use it to prevent paint and wallpaper
paste from molding but it is not legal anymore.  If they still make
"Mildoom" it would be made with a mercury substitute today and it
would probably be safe for the glue as it is was used for paint and
wallpaper paste.

However, the best solution is to remove either the warm of the wet.
Mold will not grow on dry glue so don't mix up more than you need,
but for the time when you suddenly need it without the day in advance
to let it soak, keep some in the freezer.

You can do all sorts of fun "gluesicle" projects.  Mold it in ice
cube trays or better yet put it in tiny zip lock bags to keep it from
getting freezer burned.  Then take out just what you need and put it
in the microwave on low heat.

I keep some in the freezer and have had good results but I have no
idea if this process causes any deterioration in the glue.  Ultimate
bond strength or durability might be effected.  For that reason I mix
up fresh glue the normal way for rebuilding projects when I am going
to use a lot of it.  It will not mold in just a few days.

Best regards,
Spencer Chase
Garberville, CA 95542
http://www.spencerserolls.com/