What if gesso is "too soft"?

[Matt Leahy]

Hello,
The various books I have read state that not enough glue or too much water (which I guess is the same thing) and overcooking the glue can make it weaker or lead to a gesso which is "too soft".

What are the effects of gesso that is too soft? Obviously it will mar or dent easier, or perhaps not be suited for incising lines, but are there other ill effects? My last couple panels sanded down pretty easy and I think they may be in the "soft" category. Theoretically, wouldn't a gesso that is less hard be slightly more crack resistant?

Thanks!

[Alessandra Kelley]

I was under the impression that an overcooked or weak glue leads to poor adhesion, i.e. the gesso will start crumbling off, eventually.

[dbclemons]

You could add some diluted alum to the gesso harden the glue.

[Bert Congdon]

Glue kept overnight in the frig will also be soft. Sorry to keep repeating myself, but I never keep glue and I add alum to every batch...along with a bacteriastat and glycerin.

bert

[Matt Leahy]

Thanks for the advice everyone, I appreciate it.

Bert- what type of alum do you use, and how much do you add? Is it the powdered grocery store kind?

I notice in an earlier post you write that you also add a "glug" of glycerine...is there a rough proportional estimate I can go with...your "glug" might be 20% of my mix!

Again, thanks!

[Bert Congdon]

Those are good questions. Thanx for letting me clarify. I didn't know if anybody really read this stuff.

In making say a liter of chalk and glue gesso, I will use about two or three tablespoons of glycerin. This keeps it flexible. The back of your phone book is hide glue. Without glycerin it would break in pieces when you opened it.

Formaldehyde will tan it instantly, but it may crack the gesso. Alum will tan it slowly. Tanning it means it will not be soluble in water again. If I were using it under gold leaf for an icon for instance, I would not tan it because I would want to rework the surface. Yes, it is a white powder. I purchase it in a grocery store, and I use about a teaspoon rounded in a liter.

I was given this information by a glue manufacturer from whom I purchase my glue. The experiment I did was to glue canvas to a piece of plywood, and gesso it with five coats of gesso. After a week, I soaked this under water overnight and tried to pull the canvas off. I ripped the plywoof apart getting it off, and the gesso came off like egg shells, very hard and I could break these in my fingers just like egg shells.

The bacteriastat I use is Preventol from Kremer in New York, half a teaspoon to a liter.

Bert

[Matt Leahy]

Thanks for the information Bert, I'll give your suggestions a try.
Matt