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Moderator Answer
(koo schadler)
Hello Again,
In 25 years of making gesso panels, the only time I've seen cracks, bubbles or blisters was in my first few month of making panels - once I better understood the process, I never again had those problems. So while I think glycerin is okay to add to gesso (you are right, wood moves in response to changes in RH; a degree of flexibility in the gesso can help), I also think your friend shouldn't be having those issues to begin with and needs to fine tune his process.
There are many factors to consider: the quality of each ingredient, ratios in your gesso recipe, application method, atmospheric conditions during and after. All could potentially be factors, although I don't know which, if any, are. So I'll paste below a series of general gesso guidelines. I understand they may not be applicable to your friend's process, but it's a place to start.
Regarding mold: all the ingredients of a traditional gesso panel (wood, animal glue, gesso) are water-loving. Additionally the porosity and tooth of gesso and egg tempera (both are high PVC) provide lots of places for mold spores to settle in, hide out and multiply. So mold is a not uncommon problem with egg tempera paintings (more so than with oil and acrylics). One important step is to coat the back of a painting with house paint - this prevents mold spores from settling within the panel (granted, mold can grow on a coat of housepaint, but is much more easily wiped off). Varnishing an egg tempera painting also helps control mold, for the same reasons. However varnishing egg tempera is a complicated topic (again, due to high PVC).
Anyhow, I hope some of the tips below might help your friend have fewer issues with gesso.
Koo
PROBLEMS WITH GESSO PANELS
Possible Causes and (in italics) Possible Solutions
PIN HOLES/AIR BUBBLES
1. Gesso is over heated. Never heat gesso above 135° F [57° C], about the temperature of tap water. Use a food thermometer to be sure. Keep gesso just warm enough to stay liquid (i.e. you should be able to comfortably stick a finger in it).
2. Gesso is over stirred and agitated, creating air bubbles. Work slowly and carefully. Let gesso rest overnight (refrigerated) so bubbles dissipate.
3. Gesso is applied with sponge brush or air brush (which can create air bubbles). Use a bristle brush.
4. Gesso is applied too thickly. Apply thinly. Several thin coats dry faster and smoother than a single thick layer.
5. There's too great a temperature differential between the panel and the gesso; i.e., the panel is cool and the gesso is very warm, or vice versa. Don't work in a cold space. Let panels sit overnight where you'll be working, to get panels to room temperature. Don't overheat gesso. Some commercial gesso makers measure the temperatures of panels and gesso, to insure they're the same.
6. Waiting too long between layers; this creates surface tension between layers that encourages air bubbles. Apply next layer as soon as previous one is touch dry.
Other suggested remedies for pin holes:- Add a commercial dispersant such as a teaspoon of alcohol, glycerin or commercial dispersant (Natural Pigments and Golden Artist Colors both make dispersants)- Vibrate panel, to dissipate air bubbles.- Let gesso cool & set up, then apply with a trowel.
CRACKS
Cracks that appear within a few days of gesso application may be caused by the following:
1. Flexible support. Don't apply multiple layers of true gesso to canvas or paper (unless attached to rigid support).
2. Too high a percentage of glue in your recipe, which makes too strong a gesso, apt to pull apart and crack. Adjust ratio of glue to water. Don't use less than 11 parts water to one part glue.
3. Glue is of poor quality or old. Make sure your glue is 100% collagen with a 450-480 bloom strength; is pale in color and sweet smelling. If already hydrated, do not use if it's more than a week or two old, smells rotten, or has mold on it.
4. Gesso is dried too quickly using excessive heat, a hair dryer, or a fan. Let gesso air dry in a warm but not overly heated room, away from direct heating or cooling sources. Be aware of radiators, air conditioning ducts, fans, etc.
5. The consistency of the gesso is very thick - either because it was initially made so, or water has evaporated off and the gesso has thickened. Aim for gesso with consistency of light cream or milk. Keep gesso slightly warmed. Don't evaporate water content by overheating. If water does evaporate over the course of the day, re-hydrate gesso to light cream to milk consistency.
6. Gesso is applied too thickly. Gradually build up surface with many thin layers of gesso, versus one or two very thick layers.
7. A stronger gesso, with a high percentage of glue, is applied over layers of weaker gesso, with less glue. Keep the strength of your gesso consistent.
8. Uneven surface tension on panel. Coat both sides of planar panel with equal number of gesso layers.
Cracks that appear at a later date, i.e., weeks or months after gesso panels have been made, may be caused by one or more of the following:
1. A solid wood or plywood panel without cloth attached to panel face. To prevent wood grain from telegraphing through and creating cracks, solid wood & plywood panels must be covered with cloth before gessoing. (Fiberboards do not have grain pattern and don't need cloth.)
2. Extremes of heat, cold or humidity. Store panels in a moderate and stable environment.
3. Panel is dropped, particularly on its edge. Store and handle panels with care.
4. Gesso is subjected to too much wet sanding. Don't overwork the surface.
5. Panel itself was defective from the start. Make sure wood panels are well-made and properly cured, not overly wet nor too dry. Consider using a high quality plywood, which has more stability than solid wood.
GESSO DOS & DON'TS
1. Start with a good support/panel and good support preparations.
2. Use high quality glue. 100% collagen with a bloom (gel) strength of 450-480.
3. Use good ratio of binder (glue) to inert white solid (chalk or gypsum). Neither too strong nor too weak; approximately 1 part glue to 16 parts water, combined with 1.5 parts chalk or gypsum, to create a gesso with the consistency of milk or light cream.
4. Once made, let gesso sit, refrigerated, for a day or two. This is one of the keys to good gesso. Elements come together, the chalk or gypsum becomes fully saturated, air bubbles dissipate = smoother gesso.
5. Never overheat glue or gesso. Never let gesso get hotter than 135° F [57° C] – keep it just warm enough to liquefy the glue. You should be able to comfortably stick a finger in warmed gesso and not get burned.
6. No temperature extremes. Have all materials more or less at room temperature; don't work near air conditioning ducts, fans, or heat sources.
7. Not too thick. You want gesso about the consistency of light cream or whole milk.
8. Apply thin layers. Many thin layers are better than fewer thick layers (thickness can lead to cracks and pinholes).
9. Rub in first coat by hand. This encourages adhesion between the support (panel) and gesso.
10. Don't wait long between layers. Apply next layer as soon as previous one is just dry to touch.
11. Avoid gesso build up on edges. Extra thick gesso on edges can lead to pinholes, chipping, and makes the panel harder to sand.
12. As soon as pinholes appear, aggressively brush them away or rub out using fingers. Stay ahead of pinholes! They're not inevitable.
13. Regularly check panel in raking light. It's easier to address problems like pinholes, loose brush hairs stuck in the gesso, deep brush marks, etc. sooner than later.
14. Don't sand every layer to perfect smoothness. It's a waste of time since the next layer covers the previous one and creates a new set of brush marks. Address issues #11 - 13 above, but wait until gessoing is complete before sanding to a flawlessly smooth surface.
15. Apply enough layers to achieve full opacity; then add a few extra to allow for sanding and create a more absorbent surface. A minimum of 5 layers - but 9 (or more) layers are better.
16. Take good notes: what ingredients were used, what ratios, what did or didn't work. You'll be glad you did the next time you make gesso.
