Dear MITRA team,
I’ve read through a lot of the existing threads but I do still have some question on the sizing/priming of flexible supports.
Regarding rabbit skin glue, I understand that most artists have decided against it’s use by now. But I would still like to know if conservationists unanimously advice against the use of RSG on flexible supports (like they have done with, for instance, the use of Zinc Oxide) or if it is more a matter of preference and which risks one might be willing to take with the material. I’m also not so clear on how do disadvantages of using RSG compare those of using PVA glue or acrylic size. I do understand the problems with cracking, but I’m still interested in using RSG for stiffness. I have to say, another reasons for me considering it is somewhat romantic. I like the idea of using a material that has been used and regarded as the best option for centuries. I’m aware of the faultiness of this logic though. So, please help!
My second question would be is it would be ok to use an alkyd based ground (I’m thinking titanium white, marble dust or similar, alkyd binder and maybe some oil) on top of the RSG or PVA, and under oil painting. I would do this mainly for time convenience (drying time). I wonder if succesive coats of oil would adhere well to the alkyd ground, that results quite glossy.
Finally, I would like to know how many coats of glue and how many of ground you would recomend for best results on flexible support.
Thank you so much for your great work!
Camila
Thank you for your answer.
I think I will leave the RSG be for now. I’ve also decided to use oil based grounds and just accept the long drying time. I was revisiting the information sheet on adhesives and sizes. I see that two coats of GAC 200 perform best in stiffness and oil blocking. I wonder though, as the product is recomended for non porous surfaces, if it will work best on canvas. I thought about using a coat of GAC 400 under the 200, but might that be an overkill? I see moderate stiffness is also achieved through three coats of acrylic gesso, but I have to say I don’t really like it as a material.
Stiffness is really important to me, because of the hardening of the oil film with time. So I guess my question is: which is the best combination of polymer dispersion mediums in order to achieve highest stiffness and good penetration of the medium on the support?
My next problem is that, living in Italy, I don’t have easy access to the Golden products. Import taxes and shipping costs are quite high, so I have been trying to find similar products locally. Unfortunately I haven’t jet had any success. I understand that these products are quite unique, but I still wanted to ask if you can give me any information on the matter.
Thank you for all the additional ressources, I’m looking forward to reading into it.
RSG, PVA and Acrylic sizings will all accept alkyd-oil primers if properly applied. As I understand it, the main challenges with Rabbit Skin Glue size are brittleness, hygroscopy (tendency to absorb water from the atmosphere, causing swelling) and susceptibility to microorganisms and insects. Synthetics are not vulnerable to these issues. Also, variations in glue strength are not a factor with ready-to-use, synthetic sizings.
As the OP mentions, imparting stiffness is a key benefit of RSG over most synthetics, because sizing helps stretched fabric supports stay flat after initial tension has reduced. While most synthetics aren’t the equal of RSG in this regard, Golden GAC 400 is pretty close.
More about glue strength and fomulas: when I studied art, many older painters still used RSG, and there were recommended practices that were assumed to yield better results. We would see the old Taubes “weak glue” formula sometimes, but the Ralph Mayer/Utrecht “strong glue” proportion was overwhelmingly preferred (and is still recommended by Utrecht). Glue granules were soaked prior to preparation, dissolved over a double boiler to prevent overcooking, and the most scrupulous artists would “proof” the glue on a cold spoon to ensure it would gel. Everyone applied their glue hot or warm. Leftovers would be re-liquified over heat before use.
Hi Camila –
Some thoughts on your questions:
– If you can keep a rabbit skin glue size between 40-70% RH it performs really quite well – in fact in many ways it sets the gold standard for stiffness. However as you go below or above either of those ranges the glue will shrink or go slack in increasingly dangerous degrees. These risks are rarely seen early on in oil paintings as the young oil films remain relatively flexible, with ‘young’ being counted in decades, thus the false sense many painters have that the warnings seem overblown when looking at works that are just 20 or 30 years old. But given enough time, these forces ultimately translate to a high risk for cracking, cupping, and delamination. These facts, which really are about the inherent attributes of the material, are not really in dispute, but certainly one can still use it – just be aware that you are taking on much more risk by doing so – risks that are not there with other synthetic sizes. But its allure, like the facts, also seems indisputable.
For more information on this see the second half of Marion Mecklenburg’sDetermining the Acceptable Ranges of Relative Humidity And Temperature in Museums and Galleries This composite graph, derived from his data, also is useful as a summary of how much movement various sizes and grounds have relative to humidity:
– I would caution against making your own ground if only because one cannot remotely disperse the pigments and other solids into the oil and alkyd to the same degree as a three roll mill. If its something you just want to try in order to gain knowledge, then that is one thing, but if painting anything of importance, I think it would be better to purchase a well-made ground from any number of manufacturers. Those will generally be leaner, have more tooth, and be less glossy, than a homemade recipe. Just something to keep in mind.
– Lastly, it is a common thought that by using RSG we are following something ‘used for centuries’, but its important to realize that almost nothing we are using today lines up with then – from the linen, to the oils, the sizes or the various grounds. For just a taste of the complexity, I would point to this incredible thesis by M. Stols-Witlox,
Preparatory layers for oil paintings 1550-1900
Just a stroll through the section of sizing will give a clue to the variety of materials used. And ironically, rabbit skin glue itself isn’t really mentioned in the literature until the twentieth century! See this reference from the Conservation of Easel Paintings for a great insight on that:
https://bit.ly/2XFVb0T
Even linen has changed in ways that make our current ones seem dramatically plain. Just to throw out a few quick examples, in the following article on French Canvasses since the 17th century, you find not only that most of the older ones were hemp-based or had a large percentage of hemp to linen, they employed types of weave structures that are all but gone now for most of the linen artists purchase:
http://cool.conservation-us.org/…/jaic20-01-001.html
The weave structures, in particular, would have a huge impact on dimensional stability.
And for something simply amazing, take a look at these incredible reconstructions of canvases from the past:
https://handwovencanvas.blogspot.pt/
All of this is not meant to take away from a love of tradition, but serves as a gentle cautionary tale that a lot of what we take as traditional and passed down from the centuries is intermixed with romance and often not based on facts. But the good news is that we know increasingly more about past materials and practices, and despite that ever building body of knowledge, the mystery and beauty of the art from then grows and does not diminish.
If you are able to achieve your desired degree of stiffness with acrylic dispersion primer (gesso), subsequently applying oil-based primer is also an option (instead of unpigmented sizing). Some artists do this because they like the weave-filling property and uniform opacity of gesso, but prefer the suave brush movement possible with an oil ground.
