Oil Pastel, Crayon, Casein Adhesion Tests

August 14, 2008

This procedure is based on one recommended to me by a technician at Golden Paints for testing adhesion. It’s not a perfect test, but it can still give you a good idea how well layers of different mediums adhere to each other.

As an example of this I wanted to test the combination of casein medium and wax crayon or oil pastel layering I spoke of a few days ago. The crayons were drawn on top of watercolor paper. In one area I drew different colors of crayon separated with layers of casein. In another area I drew crayon without any casein at all.

Tape on Wax

Tape on Wax

I took some thick sticky cellophane tape and pressed a small square over the area to be tested, burnishing it down firmly. I then cut three lines: two parallel to each other, and one connecting them. I made sure the knife cut all the way through the paper. Next, I peeled back the cut section to see how much of the medium came off on the tape. According to the technician at Golden, if no medium comes off you have excellent adhesion, if a small amount comes off adhesion is satisfactory but not ideal, and if the tape removes quite a bit of material then adhesion is not very good.

Wax and Casein

Wax and Casein

Wax only

Wax only

On the test that contained wax and casein, some flakes of crayon came off, but a majority of it remained. This means the casein was adhering well to the wax, but the wax adhesion to casein was only satisfactory. A test on the crayon alone showed hardly any flakes coming off. That could be that the tape just wasn’t sticking very well to the wax, although it did appear to do so.

Continuing on, I also wanted to test oil pastel and casein adhesion. As with the crayons, I had two areas on watercolor paper, one with three layers of casein between different colors of oil pastel, and another area with pastel only. I placed tape over them, sliced them and peeled the tape back. In the image below you can see the two pieces of tape. The one on the left is oil pastel and casein, the one on the right is pastel only.

O.P. with Casein

O.P. with Casein

O.P. only

O.P. only

You can see that quite a bit of oil pastel and casein lifted off with the tape. That’s not too surprising since in the other image you can see the oil pastel that had no casein also lifted off to a large degree. I know from experience that oil pastel has fairly weak adhesion. Typically they have very little oil in them, which is where most of the adhesion comes from. Different brands are not all made of the same ingredients, however, so the results may vary. These are the Pentel “Arts” brand.

My conclusion with these test is that casein binds satisfactorily to wax crayon and oil pastel. In the case of the pastels, casein doesn’t improve things too much. It might actually help as long as the casein doesn’t loose adhesion over time, so some long term tests are required to be certain.


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