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MITRA Forum Question Details

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  • Testing adhesionApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2017-08-09 14:26:38 ... Most recent comment 2017-08-11 19:14:16
    Oil Paint

    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

  • EditDeleteModerator Answer

    ​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.

    Brian Baade
    2017-08-09 14:38:14
  • EditDeleteModerator Answer

    ​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.

    Sarah Sands
    Senior Technical Specialist
    Golden Artist Colors

    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-08-09 18:50:56
  • ApproveRejectUn-ApproveUser Comment

    Hi Sarah, 

    Thanks for your answer. This is actually not the first time I've spoken to you about paint adhesion. I'm concerned about this because I've noticed my paintings can be exceedingly fragile. There are a lot of different areas on a number of different paintings where I've noticed issues, so for the sake of clarity I'll stick with the most troubling; that is where very simple methods of painting were used and problems still arise.

    These are areas where no solvent and no medium is being used, just paint straight from the tube in varying thicknesses.  It's essentially alla prima painting over and over again, each layer given time to dry and the same palette being used throughout.    

    Most areas build up enough to lose the weave of a 15 oz (unprimed) canvas.  Some areas are impastoed, some flat. I paint and repaint in many layers, so I thought this would keep the paint lean enough to always be pourous and toothy if I decided another layer was needed.

    The surfaces of these layers can range from matte and cakey, course to smooth, to more brittle, shinier impasto. All of these surface types can sometimes seem just as fragile as my more experimental surfaces, where mediums and more questionable techniques are used freely.

    Some have been dry for about a year, some a few weeks. It is hard to tell whether additional drying time is the answer. Some passages dried in a week are durable and seemingly similar areas that have been dry for six months are not. They seem weak in the same way. This makes me think a weak bond doesn't always resolve itself. So I'd like to be able to know if a good bond is actually forming, especially if it's primarily a chemical one.

      None have delaminated on their own (probably because the canvas is stretched over panel) but top layers can often be removed fairly easily with a simple piercing and running of the palette knife, peeling off cleanly, a weak, brittle layer (thick or thin) of oil paint from the layer beneath it. It takes about the same amount of force or less that it does to scratch a lottery ticket. Maybe this is normal, I don't know. But it's the separating of layers that is suspicious, as I would think a strong oil painting is bonded all the way through to the ground as one durable film.  Again, some of these layers have had months of drying time.

    The ground is a high quality acrylic dispersion made by Utrecht.  I’m now considering going to a lead oil ground to see if it helps, and also switching to using lead white. I've read the films are stronger than titanium and I use a lot of titanium.

    What is frustrating is that it seems like lean paint straight from the tube just stops adhering to itself after so many layers, sometimes surprisingly early. I thought this simple method would be fool proof. But maybe a medium is needed at some point in order to ensure adhesion to the previous layer?  I’m very wary of mediums because they can over complicate matters. Also, I've used them by the book before and the same thing is liable to happen, except the area that is poorly bonded is more skin like and flexible than flaky. 

    All of this is made more confusing because I don't know what actually constitutes proper bonding between layers and how to recognize it when it's happening. Bonding that would help ensure that the painting will survive well enough under the expected conditions that a painting is meant to endure.  

    I do not want to post images online but if you'd like to see some of what I'm talking about, I can send them via email.

    Thanks again, I know this was long but I can't tell you how much I appreciate this feedback.

    2017-08-10 13:11:05
  • EditDeleteModerator Answer

    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.

    Matthew Kinsey, Utrecht Art Supplies
    2017-08-10 22:49:04
  • ApproveRejectUn-ApproveUser Comment

     Thanks, Matthew.  Yeah, I've experienced this with alkyd mediums, as have many artists that I know. I have noticed, however, that some bonds never get better as they dry, or if they do, they improve very little. I’ve noticed it's the bonds that become firm within a week or two that are generally the ones that stay strong, but maybe I'm wrong there.

    I've just read some of the basic concepts of commercial paint application. I know they must have different standards, but the concepts of dispersive adhesion such as “wetting”, “roughness” and differing surface energies must apply to oil painting as well, right? If these are universal to all types of painting, is it possible I'm getting air pockets and bridges between lean paint layers that are preventing adhesion?

    2017-08-11 13:50:56
  • EditDeleteModerator Answer

    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.

    Matthew Kinsey, Utrecht Art Supplies
    2017-08-11 18:22:13
  • ApproveRejectUn-ApproveUser CommentI see. Great, thanks for all the help.
    2017-08-11 19:14:16

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