Blocking Support Induced Discoloration
Support Induced Discoloration or SID describes a phenomenon that can occur when the acrylic appears to change in color upon drying. It usually takes on a yellow, orange or brown tint, due to impurities in the substrate being drawn up into the acrylic film. The discoloration occurs while the paint or medium is drying and curing and should not continue or happen after the film is cured. This can be found in supports including woods, hardboards, particle boards and some canvas and linen supports. These impurities can include glues, resins, sizing and any soluble materials in the substrates. SID is only applicable when painting with acrylic paints and mediums and most noticeable in thicker applications of clear or translucent mediums and gels and in opaque and semi-opaque pastes. It can affect some paint colors as well.
In 1989, GOLDEN partnered with the Art Conservation Department at Buffalo State College to try to understand what was causing a fast yellowing or browning of a matte acrylic film on canvas. Through an error in the testing of a small corner of acrylic gel becoming detached from the canvas and remaining clear, it was determined that the substrate was causing the discoloration and not just the acrylic film changing color on its own. The term Support Induced Discoloration or SID for short was then coined. (1)
Our recommendation to block SID on these substrates has historically been GAC 100, but recent testing has shown that our Gloss Medium is more effective and we have changed our recommendation to reflect that. As you can see in the image below, applying 2 coats of Gloss Medium directly on the support before applying any other product, is most effective in blocking SID from the thick application of Regular Gel Gloss. In the case of OPEN Gel Gloss, which has a very slow dry and curing time, the wet product lingering on the surface allows more impurities to be drawn into the gel and the Gloss Medium is less effective. This is a “worst case scenario” test, as we only recommend painting with OPEN products thinly. It should also be noted that Gloss Medium and GAC 100 do not seal wood, they really just serve to block SID. Washing the canvas before painting can be somewhat effective, but usually this results in a wrinkled fabric and it can be hard to rid of all of the wrinkles, even when stretching. Washing linen was less effective to stop the Support Induced Discoloration. (2)
From left to right: No sizing and Gesso only, One Coat of GAC 100 then Gesso, Two Coats of GAC 100 then Gesso, One Coat of Gloss Medium then Gesso, Two Coats of Gloss Medium then Gesso
More information on Support Induced Discoloration can be found here in the video Support Induced Discoloration (SID), what is it? and how to minimize it: https://vimeo.com/273304693 and on the goldenpaints.com website here: https://www.goldenpaints.com/technicalinfo/technicalinfo_prepsupp or by contacting GOLDEN at 800-959-6543 or by emailing us at help@goldenpaints.com.
(1) , (2) Hamm, J, Gavett, B, Golden, M, et al., “The discoloration of Acrylic Dispersion Media”, Saving the Twentieth Century: The Conservation of Modern Materials, Canadian Conservation Institute, 1991, 381-392