Hi Koo -
Thanks for the questions. I think the big difference you might notice between SID caused by acrylic-based products, vs other waterbased ones, is the fact that the water within acrylics contain surfactants which act as soaps. This gives the acrylics a much stronger ability to dissolve and carry away not simply purely water-soluble impurities, but an assortment of other materials that the surfactants are able to latch onto. Think of washing clothes or dishes in cold water alone versus with a detergent or soap. Its a crude analogy but it gets the point across.
As to your specific questions, I try to address them below:1. The “Rigid Supports” article posted on this website, when discussing hardboard, says “SID will occur if the oily or resinous material migrates through the size or seal and stains the ground or paint layers”. My understanding is that oils and/or resins in or atop engineered wood-based panels are stable and cannot move or migrate – they are polymerized or cured and can’t go anywhere. Yes, no?
While some of these might be polymerized and stable, there is manifestly a lot of other, similar materials that are mobile and can cause discoloration - as any of our SID testing shows, especially on hardboard panels. While considerably less, one can also get discoloration from things like birch plywood, linen, and cotton canvas. We do not, however, get SID when using MDO plywood, which has a resin-impregnated paper coating which seems to act as an effective barrier.
2. My understanding is that SID is caused by materials in the wood itself (tannins, dirt, sap, starches), drawn up into the gesso by water based grounds and paints. Yes, no?
Yes. Although when applying acrylic Gesso alone onto a wood or canvas support, one rarely if ever sees significant SID as the drytime is so rapid that there is not sufficient dwell time for moisture to penetrate into the substrate in a significant way. We also almost never see SID in fully pigmented paints, where the pigments mask and help block the discoloration. Where we see SID most is in clear or lightly pigmented glazes and gels, especially any that are slow drying.
3. Has there been testing to see if SID occurs in a traditional gesso ground and/or egg tempera? I’ve done a couple of tests myself (painting a word on a panel using GAC 100, applying traditional gesso all over, seeing if the word later appeared) - very little to no SID appeared. Could the very high solid content (percentage of chalk) in traditional gesso inhibit SID?
We probably do more SID testing then any one else but unfortunately have not done any testing specifically around the use of traditional gesso. That said, it would not surprise us if little SID was evident. In this case we think both the high solids nature of the gesso, as well as the lack of surfactants, would make the occurrence of SID minimal or nonexistent. That said, we would certainly be open to running some tests and could perhaps get some of that going after the first of the year.
4. The various products for blocking SID (Archiva-seal, GAC 100, PVA) are all polymer based and designed to sit under acrylic or oil grounds, not traditional gesso. I tried a crosshatch adhesion test of traditional gesso atop GAC 100 and adhesion was not great. If SID is a concern with egg tempera, any ideas for how to block it? I presume a layer of cloth applied with PVA to a engineered panel would do it, but is there a simpler option?
A couple of notes. We have not seen any testing concerning the effectiveness of PVA in terms of blocking SID. We do know that different acrylic polymers will perform differently, and so hesitate to recommend any material in general. Even within our recommendations, we have recently confirmed that our Polymer Medium does better at blocking SID than GAC 100 and that could ultimately alter our recommendation.
So, along with testing SID and traditional gesso, we can certainly look at how effective PVA is as well and share the results.
We agree that adhesion of traditional gesso onto GAC 100 does not appear great. A few things you might look at, considering this is on panel, is using a pigmented white shellac, such as BIN, which can be effective against SID, especially in multiple coats. I am not sure if an egg wash would be effective n=but it might be, and could perhaps be compatible. Again, something we could look at. Finally, as stated earlier, it might simply be that the issue of SID and Egg Tempera is a nonissue and that if you see no discoloration in the traditional chalk gesso, then there would be no reason to fear that it would appear suddenly later on.
Senior Technical Specialist
Golden Artist Colors