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In making gesso, my experience is to first establish a ratio of glue to water, my preference being 1 pt. glue to 16 pts. water (1:16). While I've always believed there is some variability in that ratio (my guesstimate, from 1:12 to 1:20), a past MITRA question clarified that, given the complexity of glue, the variability can be greater than that (i.e. one paint company recommends a 1:5 ratio) - it all depends on glue factors.
Anyhow, the second relevant gesso ratio is glue water to chalk or gypsum (whiting). The most common ratio I see (and use) is 1 pt. Glue Water: 1.5 pts. Whiting. This generally yields a gesso with the consistency of light cream or whole milk, ideal for applying gesso. Too thick a gesso is more apt to crack.
The Mt. Athos monk with whom I've conversed in the past is asking another question, relative to the above. His gesso recipe, like mine (1 glue: 16 water + 1 glue water: 1.5 whiting) is yielding a gesso with a heavy cream or pudding consistency. He is asking which is more relevant: staying with consistent ratios (even if it yields a thick gesso), or shooting for a gesso consistency of light cream (even if that means adding less whiting). My gesso never yields a pudding-like gesso, so I'm puzzled why he's getting that. Regardless, I would say consistency is more important - applying pudding-like gesso is not good. But then again, gesso is essentially a high PVC paint, and if one too dramatically alters a paint's PVC, isn't that problematic?
I realize all this is complicated by variabilities in ingredients and measuring, environmental factors, etc.; and there is not a simple answer. Nonetheless, I welcome general thoughts on the above, as well as a response to the specific question of which is more important: staying with consistent gesso ratios, or adjusting a ratio (perhaps dramatically) to yield a thinner gesso consistency.
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What is more important than the water to glue ratio is the glue to pigment ratio. Since water evaporates what remains of the ground is only the glue and pigment. The greater the content of glue in relation to pigment the more the ground exhibits the hygroscopic behavior of the animal collagen glue.
Hi George. The raises a question for me: Does traditional gesso have a CPVC percentage? I am guessing that glue (unlike linseed oil, egg yolk, etc.) is too variable to have a predictable PVC; but what if we presume a 450 bloom strength glue? Would that have a known CPVC for traditional gesso?
This is an inconsistency that's confused me: on the one hand, when making paint, we begin with a ratio of binder to pigment, i.e. its PVC; and, for water-soluble paints, the water isn't relevant to the ratio since it evaporates, as you note.
On the other hand, when making gesso - essentially a high PVC paint - most (all?) recipes do not begin with a ratio off binder to pigment; instead they start with a ratio of binder (glue) to water (the thing that evaporates out); followed by a ratio of glue water to whiting.
The water component, which is essential but also variable, seems to obscure the binder to pigment relationship in gesso. Is there a reason gesso recipes don't start, as do other paint systems, with a binder to pigment ratio? Is it because of the necessity of dissolving glue in water to make it a workable binder? Or due to the variability of glue?
I have a recipe that works predictably and realize it's not necessary to determine my gesso's CPVC. Still, I would like to understand better; and to understand why (as discussed in a previous post) gesso's CPVC does vary so much from one recipe to another. As to that last point, I think it's because of the variabilities in glues and types of surface (more or less absorbent) that a painter wants to work on. However this results in a lot of built-in uncertainty to gesso, which makes it hard for a gesso maker who runs into issues (such as my monk friend) to diagnose the problem. If there isn't a CPVC that can be determined for gesso, how does he know if his gesso is too thick because his CPVC is wrong, or because his chalk is behaving oddly?
Thanks for any comments. Koo