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(ssands)
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's Determining 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:
https://www.justpaint.org/wp-content/uploads/2015/05/3.jpg
- 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.
