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Question asked 2019-07-11 10:26:10 ...
Most recent comment 2019-07-11 16:00:49
Grounds / Priming
When I first became a tempera painter, I experimented with many different recipies for traditional gesso (gleaned from various books – there was no on-line world back then!). I got both good and bad results (some gessoes developed cracks). Eventually, after much reading and experimenting to better understand the properties of traditional gesso, I arrived at what seemed a solid recipe based on ratios versus specific measurements; it's always yielded a good, reliable surface, and I've heard from many artists with whom I've shared the recipe that they too use it with success.
I've found that a mix of 1 part glue to 16 parts water, then 1 part of this glue water combined with approx. 1.5 parts chalk or gypsum, yields a not too soft, not too hard ground. However there is some variability in the water to glue ratio – more glue in the mix yields a harder ground, less glue a softer ground. So an artist has flexibility in the recipe depending on what sort of ground he or she wants to work on. On the other hand, in my experience, if one strays too far outside certain parameters in the water to glue ratio, the resulting ground is either too hard (anything beyond about 1 part glue to 12 parts water = too much glue in the ground) and is prone to cracks; or too soft (anything less than about 1 part glue to 20 parts water = too little binder) and the ground becomes friable.
Recently I read a gesso recipe from a paint tech, whose knowledge I trust and respect, which recommends a ratio of 1 part glue to 5 parts water. I was surprised by this number; it seems way too much glue to me, and in fact I have a panel made from this recipe that dramatically cracked (see attached image). 
Gesso Panel cracks.jpg However the tech feels confident that a 1 glue to 5 water ratio is reasonable for gesso, and also has panels made from this recipe that haven't cracked. (FYI, I understand that glue is not the only reason gesso can crack – changes in temps, humidity, wood grain telegraphing through, dropping a panel on its edge, etc. also can cause cracks…)
I've always felt pretty confident of my understanding of true gesso – that too much glue in the mix is problematic, and anything beyond about 1:12 or maybe 1:10 tops is too much glue – but now I'm wondering if there is more flexibility in the ratios for true gesso than I realized. Any thoughts? I want to make sure I'm understanding gesso correctly.
Thanks, Koo Schadler
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Moderator Answer
(brian baade)
Koo
This is very difficult to write about with authority, as
each glue may have a different bloom strength and therefore your 1:16 may be a
different “strength” than another persons, depending on their glue. This is one
of the reasons why I always use the same brand of glue in my classes. )Not
because it is inherently superior, but that I understand how it will perform
for the students. That ratio is about 10.5 parts water to glue (ml to ml , I do
not weigh the glue for this recipe).
On the other hand, there is some degree of flexibility where
the gesso will perform as hoped, and not be too powdery or brittle. As you have
done, you sort of need to work this out depending on your choice of glue. If
you get you glue from a source that lists bloom strength, this is a lot easier
to quantify and to repeat.
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Moderator Answer
(brian baade)
I am sure that one could work out exacting parameters with
testing a particular glue AND a particular grind of gypsum/chalk allowing one to
quantify at what point failure (on either side) would occur. I have not done
that experiment. There is certainly some wiggle room as to how hard or soft your
intended ground will be but I am not sure just how wide that margin is. I know of no
study that has attempted this. Perhaps this was done by one the more
technically minded materials and techniques gurus (eg Mayer, Toch, etc.) but I
know of no published study.
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Moderator Answer
(george o'hanlon)
There are many factors that affect the ratio of glue, water and pigments in ground recipes, such as the particle size, distribution and shape of the pigment and the gel strength of the animal glue as have already been discussed. A few more factors to add to this list is the composition of the collagen and more importantly the moisture content of the glue which can range from a few percent to 20 percent. The latter has a major impact on the glue strength and ratio of water to dried glue leading to the variable formulas encountered in literature and in practice. I’ve written an entire article on the subject at the Natural Pigments web site:
Preparing Hide (Collagen) Glue
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Moderator Answer
(brian baade)
Thanks so much George. Can you post a link to that article here?
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Moderator Answer
(george o'hanlon)
I've posted the link to the article in my comment above.
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