Question asked 2017-08-09 14:26:38 ...
Most recent comment 2017-08-11 19:14:16
I was wondering if you have any advice/ways of testing adhesion between oil painting layers? I have done a diy cross cut test with a razor blade and masking tape, but if I go by that thicker passages and impasto pretty much always fail, so it seems like overkill. On the other hand, a fingernail seems kind of weak, because I've not been able to scratch layers that I otherwise can peel or sand off relatively easily.
Answers and Comments
I think that it would be best to leave this question for moderators who work for the fine art paint industry as they deal with this testing on a regular basis and will be aware of the most current thoughts on the subject.
Your question raises many others before really being able to address it. When you see failure in the thick areas, is it a clean cleaving between layers, or failure within that actual paint itself? What is the substrate? How long have the films dried? Are you using mediums, solvents? Different paints will take different lengths of time to fully cure and reach maximum adhesion, so knowing some of those parameters is important to knowing if you are testing too soon. It would not be crazy to need to wait 6-12 months or, in thicker applications, even years, before full adhesion is reached.
Would also be good to know why you suspect layers are not adhering? Adhesion issues in the short term are not a problem for most best practices when paints are allowed to cure naturally, but it sounds like you might be putting the films through some sort of extreme stress early on or trying to push techniques or processes into areas that oils are not well suited for. Linseed oil in the end is not a very strong adhesive and can only take so much stress or strain before breaking - which might be mistaken as a form of adhesion failure - versus true delamination where one layer truly comes free from another.
Finally, just in general, crosshatch adhesion or the simpler "x" field test are really the standard for interlayer adhesion testing for something like this. Other alternatives, which involve embedding a material between layers, or gluing a metal disc to the surface with epoxy and pulling these strainght up are meant more for pure test panels for specific applications and not an actual work in progress. So you are on the right track - but it might be that the test is too strenuous for the stage at which it is being used.
Look forward to more information to see if we can help.
Senior Technical Specialist
Golden Artist Colors
Sorry to join the discussion late. I suspect what's been described is the result of upper layers skinning over quickly while the paint beneath is permitting movement. The fact that the problem manifests late in the process is a telltale sign. I have seen this before, when artists have layered paint with a fast-dring alkyd medium on top of touch-dry (but still uncured) oil colors that have a high oil content. The top skin can be scratched off easily at first, but as it stiffens and the lower layers proceed toward a full cure, the film becomes more mechanically durable. Whether the final bond is optimal is another matter, but the tendency to easily scrape away from lower layers does lessen as the film dries.
I think the addition of a medium that dries at a moderate rate, especially in the upper layers, would make a positive contribution to this process. This would help top applications keep up with dimensional changes in lower layers and should promote better adhesion in complicated layering. There are also other advantages to using a medium in layered techniques, in terms of handling, and achieving better control over the appearance of colors.
Remember, artists' oil colors are not utility coatings, they are specialist materials for a sophisticated historical craft. Unlike acrylics and other modern materials, oils have largely not undergone heavy redevelopment to improve on traditional performance. (Paradoxically, many experimental artists favor traditional oils and actually love them for their limitations as well as strengths.) It's the nature of oil paint to behave this way when colors dry prematurely over semi-wet paint. I really think a medium with a moderate (not fast) drying rate will help enormously.
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