Thread: Gesso problems
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Old 25-11-17, 07:12 PM
Koo Schadler's Avatar
Koo Schadler Koo Schadler is offline
Tempera Painter
 
Join Date: Dec 2003
Location: Alstead, NH & Zirahuen, Mexico
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Hi Chris,

I can think of three possibilities why your gesso surface misbehaved:

1. One factor in traditional gesso’s absorbency is the amount of glue in the recipe. A typical gesso recipe is:

1 part glue +16 parts water +1.5 parts whiting (chalk or gypsum).

This yields a not-too-hard, not-too-soft ground. More glue in the recipe creates a harder, less absorbent gesso; more chalk (or gypsum) in the recipe yields a softer, more absorbent gesso. Gamblin’s products are generally very good, so I don’t doubt the quality of the ingredients – but perhaps their mix produces a hard gesso? I don't know. You could ask Gamblin if they’ll share their gesso ratios with you, to determine where it falls on the hard/less absorbent versus soft/more absorbent spectrum. Or, try adding a bit more chalk or gypsum to their mix and see what kind of surface results.

2. There are two ways egg tempera adheres to traditional gesso. One is via “dispersive adhesion”; molecular changes cause two disparate substances (egg tempera and gesso) to adhere. Dispersive adhesion is hard to explain; I don’t really understand it, I don’t think you need to understand it per se, but if interested you can look it up – suffice to say it is considered the most important kind of adhesion.

There is a second kind of adhesion that I think is also very important for egg tempera: mechanical adhesion. Think of Velcro or sewing…basically two disparate materials interlock or mesh with one another. Both egg tempera paint and traditional gesso have a very high “Pigment Volume Concentrate”, or PVC (see this link for more about PVC: http://www.eggtempera.com/forum/show...trate#post6825). The irregular surface (on a microscopic level) created by a high PVC ground and paint allows for the two substances to interlock or “knit” together.

Mechanical adhesion not only improves the long-term adherence of paint to gesso, it also helps the paint to behave as it’s applied. A ground with a bit of tooth (even if that tooth is imperceptible) lets the paint sink in and lock into the ground (and thus not be as prone to lifting with subsequent layers of paint). The porosity of a more open ground also allows water to sink in more readily, and this too makes it easier to manipulate the paint and build layers (since water accumulating on the surface can cause ET paint to continually dissolve and lift).

When you scraped your surface smooth with a cabinet scraper, you removed ALL the surface irregularity in the gesso. The shine you pulled out proves this: your gesso surface started as the equivalent of frosted glass (matte) and you smoothed it out to the equivalent of polished glass (high gloss). Thus no mechanical adhesion could take place, which made the paint misbehave and not adhere well.

I know one often reads that you should polish traditional gesso to the smoothness of ivory, and it’s true that traditional ET painters did so – but primarily this was for gilded areas: to emulate the appearance of solid gold, underlying layers must be perfectly smooth. But in areas that are not going to be gilded, there is no need to create a high level of gloss. Use sand papers of increasing grits (I start with 180 grit and end with about 400) ‘til you don’t see any more brush marks (or whatever tool you used to apply gesso), and until all you see are the barely visible scratch marks of 400 grit sandpaper. Then finish with a light water sanding (I prefer to use my fingers). That should be enough to create a beautifully smooth surface - but not a polished one with a high gloss that’s unreceptive to paint.

3. The final question I have is about the slight yellowing you saw after you rubbed the surface with water. I can think of two things that might cause that: (1) SID, or “support induced discoloration”, when water traveling in and out of the gesso causes impurities in the wood-based support to migrate up to the surface; and (2) as you wetted and rubbed the surface you lifted up chalk, the animal glue within the gesso migrated to the surface, which affected the glue to chalk ratio (and made a harder gesso). Both of these scenarios would occur only with a lot of water moving in and out of the gesso, which it doesn’t sound like you did - so I don’t think they are likely. I’m just trying to puzzle out that bit of yellow…

Hope that helps.

Koo

Last edited by Koo Schadler; 25-11-17 at 07:17 PM.
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