Monday, March 26, 2018

340: Why Paintings Crack, Part 2

Closeup of Summer Sky, 48" x 48"
–PAINTINGS with thick applications of paint are especially prone to cracking. So I've decided I need to take strong measure to ensure my paintings, which are distinctive for their thick texture, will not crack well into the next century.

I won't treat the subject of painting layers in the correct order, with the right proportion of oil, as it is outside my expertise. My paintings are painted alla prima, that is, in a series of sessions while the paint is still wet. When it begins to dry, even just lightly skimming over, I am done painting. The technique I developed doesn't allow for adding layers of paint over previously dry layers.

With that in mind, there are two aspects of creating a solid paint film on a canvas. The first consideration is to treat the canvas so the oil does not soak from the paint layers into the raw canvas. This means adding an isolating layer. This can best be done by coating the canvas with an acrylic medium. My preferred method is to use a bristle brush to apply a thick layer of acrylic gloss medium. In addition to creating an oil impervious layer thereby protecting the canvas from the effects of the oil, the acrylic medium helps stiffen the canvas making it less movable while still maintaining flexibility.

Once impermeability is established you need to create a surface that oil paint can adhere to. The best bond would be a chemical bond. But there are few surfaces oil will chemically bond with. One such material is copper, especially with lead-based paints. Copper will chemically cross-link with oils and create a nearly uncrackable bond. I have seen paintings by Carl Block which were painted on copper panels that are in perfect shape nearly 200 years after they were painted.

 Few can afford to paint on copper panels these days. So we have to content ourselves with creating a solid physical bond. A layer of acrylic gloss medium has no tooth for the subsequent layer of oil paint to adhere to. The only way an acylic layer and an oil layer can bond is by mechanical means. This means that the acrylic has to have tooth, or actual physical properties that intertwine between oil and dried acrylic. Think of this mechanical bond as a kind of interlocking mechanism like velcro, but on a microscopic level. The acrylic layer must provide something for the oil to latch onto. This requires adding something to make the acrylic emulsion porous. Essentially the requirements are contradictory–you need the canvas to be protected from absorption and the oil painting side to have an element of absorption, but not so much that it will absorb into the canvas material.

Acrylic gesso (or more aptly named acrylic ground) is made up of acrylic polymer and marble dust. The marble dust creates just enough permeability that the oil can latch onto the surface. In the absence of such permeability, the oil paint would just sit on top of the emulsion and would eventually delaminate (that is, peel off like an old band-aid).

There are other ways to create permeability. You can add a coat of acrylic matte medium (matte mediums have a marble dust-like additive like aluminum hydroxide). This creates a sandy, gritty surface–perfect for applying oil paint.

In summation, you need to isolate the canvas with an acrylic medium. Then add an acrylic medium that will cross-link to the subsequent acrylic layer and provide tooth for the oil paint to adhere to. With most painters, these two processes can be done by coating the canvas with three layers of quality acrylic gesso. In my case, where I want to draw on the canvas with acrylic markers to clearly define my composition, I will seal the canvas with acrylic gloss medium, paint on a layer of acrylic Mars Black, draw the composition with white acrylic markers, and then make the surface porous by adding a layer of matte acrylic medium toned with Golden Colors' Heavy Body Red Acrylic (which compositionally is very similar to acrylic gesso).

This might seem like an elaborate process, but it is one that will ensure that my highly textured painting have the highest chance of not cracking well into the next century.

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