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I was wondering if anyone knows of research on the load traditional gesso puts on a panel/cradle when it has fully dried, ie. when it shrinks thereby putting pressure on the panel causing it often enough to become concave? For a few years I have been fumbling along to create larger scale panels for my traditional egg tempera painting (30" - 45" and up). Currently I use a combination of wood and aluminum cradle glued to a tempered hardboard face. My success is fairly average. All my cradled panels have some amount of concave warp. Many are still usable but some are too curved to tolerate. My research now is to consider a more serious engineering approach. For example, 80/20 has a calculator to figure out the beam deflection for aluminum extrusions based on how many pounds of force is applied. If there was an understanding of how much force, say 8 average layers of traditional gesso put on a panel, perhaps one could calculate a material for the cradle that could sustain such pressure. I would love to hear if anyone knows where to look for this kind of research.
A related question would be: Could adding more water to my gesso recipe make the glue less powerful, therefore adding less pressure to my panel when it dries and shrinks? I generally use the Koo Schadler / Daniel V Johnson ratio...
Link to the 80/20 beam deflection calculator
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My paintings are fairly small, I'm not experienced with large panels - so, unfortunately I cant be much help. Here are a few thoughts that may or may not be relevant to your situation.
Given the many variables (glue bloom strength, concentration, number & thickness of layers, etc.) I would think it pretty difficult to calculate the force of gesso. Even if you could, the unpredictability of RH absorption can still leave a wood-based panel open to movement. Small panels also warp if the coating on the two planar sides are too disparate.
What I've found the most helpful to reducing warp is to make sure both front and back have equal numbers of gesso layers. I recognize you can't achieve that if a panel is cradled (frankly, as I believe we've discussed here before, I think cradles both solve and create problems in a panel). Are you willing to work with plywood? If you were to use ½" thick minimum (¾ to 1" would be much better, albeit heavier); apply cloth to both planar sides; then apply an equal number of gesso layers on both sides; coat the back with house paint; apply ET paint to the front - in my experience that's a pretty stable panel.
Another option is honeycomb aluminum. If you were to attach cloth to the front using BEVA or a PVA glue (use a tight weave, so the glue doesn't travel to the face of the cloth, which could affect gesso adhesion), traditional gesso would adhere to the fabric. I've tried it; it's a light weight, stable option - however not having a wood-based support does decrease the absorbency of the painting surface, even with traditional gesso; the water in the paint runs into the aluminum, so it stays more present in the gesso/paint layers, which makes everything slower to dry. So if you have watery working methods, ET won't behave the same on aluminum; however, if you work drybrush, it behaves pretty well.
I'm not sure about the answer to your second question. What I can say is that the water in gesso evaporates out, leaving behind the same ratio of binder to pigment as when you started - so adding more water doesn't weaken the glue, per se; it would make for thinner layers, so in that sense they would be less strong (but also less thick, and hence less absorbent). Also be aware that adding a lot of water can make the binder (glue) so attenuated that the solid particles (chalk, gypsum) are somewhat under bound.
If you figure something out, please let us know - I'd be interested.
I’ve done 30” sized ¾” baltic birch panels in the method you describe with good success. It was my attempt to reduce the weight of larger scaled panels that sent me down the road of using cradles. Regarding the problems common with cradles: at first, I had fantasized that the minimum amount wood in my cradle would minimize problems in the future (Cradles I’ve used are prefab strainers that combine aluminum with wood.) As I come to understand the properties of wood, I imagine even the inch or two of wood in my cradle could cause future warping etc :/
Therefore, my new fantasy was to use higher tech cradle materials with no wood, or just a wooden face (plywood or hardboard). And with some kind of reversible adhesive (VHB tape?) that sticks the face to the cradle.
I have struggled to find a source for honeycomb aluminum in New York that would actually sell me a single sheet to try, although one must exist. Does anyone have a source?
It was also recommended to me (perhaps on this forum?) that there were dangers of adhesion putting traditional gesso over linen glued down with pva… What you suggest is that as long as the adhesive is not coming through the linen, the gesso should stick to the linen? This may be the clue I need for future experiments.
For what it’s worth, I’ve managed put weight on a recently gessoes and warped cradled panel and bend it back, fairly close to planar. For the time being this will allow me to keep painting!
Thanks Koo for continuing to generously share your findings,
Even if a cradle is wood-free, as long as the panel itself is wood-based it will want to move - and if it's attached to a cradle that doesn't move, than that disparity creates tension and will create cracks in the gesso.
There are complicated cradle constructions that attach to a wood panel, but only loosely, so the panel can still move a bit in response to RH, but not flex - I have one (made by a company called Studio Products, many years ago); it's quite complicated, worked only moderately well (came unglued in parts). Still, that sort of cradle is an option, albeit labor intensive. See this link: https://en.wikipedia.org/wiki/Cradling_%28paintings%29
I don't know where to get honeycomb - I got a sample by calling an aluminum sales rep, many years ago. Have you tried contacting signmakers?
You are correct, traditional gesso doesn't adhere well to synthetic polymers (such as PVA glue). So you would have to be very careful to have enough glue on both the panel face and fabric back so that every inch adheres; yet not so much glue that it soaks through to the fabric face. It would be tricky, as you really do want good, consistent adhesion between every inch of the fabric and panel.
You'll often hear BEVA film recommended as an adhesive for aluminum and fabric, but in my experiments I found one needs to get the exact right heat setting, or the BEVA won't adhere properly - using an iron definitely didn't cut it. In my estimation you need to use a temperature control dry mount press to get the BEVA to properly, consistently activate and adhere.
Panels can have amazing memories - I'm glad your panel is flat again, but be aware it may or may not stay that way, once fluctuating RH enters into the picture. Good luck and keep us apprised.
Hello Koo and Mitra folks,
Just an update that my warped, cradled hardboard, gessoed panel (44"x32") which I managed to flatten under weight, returned to it's cupped shape after I applied a couple of relatively wet under painting layers. Then, later, my framer did an experiment by laying the panel over a strip of wood that traversed the middle, kind of like a fulcrum on a see-saw, and he claims the piece has returned to flat under its own weight. I haven't see this in person, and it seems hard to believe. Regardless, I can confirm that the panel does have a kind of life of its own. I think I can live with the panels moving around a little bit, as is natural, but my biggest fear is one day a painting ships to another country or city and it warps into a cylinder or sphere!
What is the probability that a cupped panel could become doubly or triply cupped when placed in a new environment?
My new experiments will be two-fold: 1. Return to Baltic Birch with equal cloth and gesso on both sides. It's just that the recommended 1" is too heavy at large scale. I will try 3/4" first. 2. find or build a composite material of aluminum and hardboard masonite.
Perhaps an aluminum cradle with hardboard on both sides of the cradle and gessoed on both sides. I wonder if the minimal wood-like properties of the hardboard and the stable properties of the aluminum togher would be very stable. More soon... - eli
Thanks for the update, Eli. I've had panels that warped (due to ueven coatings on back and front) that I tried to amend by remoistening the wood, then applying weights to flatten it (it did flatten), then applying additional coats to the convex size to even the tension, then reapplying weights to encourage the panel to stay flat...only to find that, after all that effort, on a humid day the wood bent once again! So yes, wood has a very good memory. I don't think a panel will necessarily curve much, if any, beyond the original bend, nor grow new convexities - but with the slightest encouragement (humidity) it will want to return to that original bend. I'll be interested to hear if your framer's fix works longterm; frankly, I'm doubtful.
My experience is that ¾" plywood is pretty stable, not quite but nearly as good as 1". I'm still dubious about applying wood based panels to an aluminum cradle - hardboards too swell and change dimensions with humidity, and if it can't do so (being attached to an unmoving base of aluminum) those tensions will travel into the gesso and paint layers. It may cause cracks (hairline generally, to start) quickly, or they may take years/decades to appear, but I think the difference in the two materials will at some point have consequences.
Sigh....let us know how it goes. Koo
I take some solace in knowing that the panels are not too likely to develop even worse curvature than they already have.
And I'll report back to you regarding my experiments with hardboard mounted on aluminum when I get around to it. My thinking is that the aluminum cradles will indeed have some flex to them to accommodate movement in the hardboard. Flex will be especially present at the scale I am working with (35" - 45").
Another Idea I had was to screw down a cradled panel from behind, so that when the gesso layers are drying and shrinking on the face, the cradle will not be able to warp. Likely this would cause cracking in the gesso, but maybe it would work the tension out somewhere else...
Just a note to report back on my panel project. I have had success using true gesso on cradled hardboard by srcewing the cradle down to a large piece of plywood for the duration of gesso application and drying time. This holds the piece in place making it unble to cup when the gesso surface shrinks. I am not sure where all the tension goes, but the panels are flat and there are no cracks.
I had similar success with this method using 3/4" Baltic Birch. I screwed into the back to hold it in place for gessoing the face, and used clamps to hold it when gessoing the backside. I did a side by side comparision on 90cm x 90cm 3/4" plywood, one held down in this fasion and one not. The peice that was not held down in anyway, despite gessoing both sides, warped substantially (a straightedges across the plane registers a 7mm gap in the worst spot.) The clamped piece had a small warp about 1mm at one end that could have been in the piece of wood to begin with.
Just wanted to let you know. Thanks for your help, Koo especially. Cheers! - eli
This is great information - what a good idea to screw and/or clamp the boards while gessoing, and I appreciate you measuring the bend in the boards to clarify the effectiveness of the method. If I ever get around to doing an update of my Egg Tempera book (which I hope to; there's so much more to say!) I'll include this information, for sure. Thanks for reporting back to us.