Thomasin,
I don't know what is a double boiler, but here is the traditional way I learned to heat the RSG : in french it's called " au bain-marie". You don't need to make the water boil, the RSG souldn't exceed 60 celsius degres, or it will loose it's qualitys.

Marina, thanks for posting that picture. That's exactly the kind of "high-tech" double boiler I've always used . . . If hot water from the tap is at least 150F, that's adequately hot to liquify RSG that has cooled to a gel, using the "system" pictured.

RSG question
 

Hello everyone,

I have been using modern acrylic gesso up until now, but decided that I should try the older methods. So I made a mixture of RSG for several recent canvases. Not having done this before, I realized (too late) that I had made the mixture too rich. The canvases look fine now. But I am concerned that the glue might run into problems this summer when the humidity is higher. I wonder if there is a danger of mold. Is this something I should be concerned about? Is there anything I could do at this point to prevent future problems? I am wondering how forgiving the mixture is if you don't do it perfectly. Does anyone have previous experience with working with glue.

P.S.
I plan to use a lead based ground (Holbein) before I start painting.

Mary, future problems with RSG have less to do with mold or varmints than what a too-thick application to raw canvas does. Too much glue is brittle, and that's not good under oil paint.

The primary purpose of the size is to isolate canvas fibers from absorbing oil, solvents, and vehicles in the paint. Ideally, a solution is about eleven tbsp. of dry glue to a quart of water. The quality and fineness of the dry glue is a factor.

First, cut that glue solution by half, to give the canvas a "drink". When that's dry, apply full-strength glue to the painting surface only. Hopefully the "drink" will keep the canvas from becoming saturated through and through with full-strength glue. When the solution starts to cool and gel on the surface, smooth it out with the palm of your hand. When dry, you may wish to lightly sand it with fine (220) sandpaper before applying the oil prime.

At this point, you'd be better off to wash the glue you've applied from the canvas (that can only improve things) and start over, so as not to have "overdone" the glue application. Hope this helps.

Thanks so much, Richard. I put the canvas under a stream of water and used a small scrub brush to remove as much glue as possible. I plan to let it sit awhile to soak the glue and repeat the process. How will I know when I have gotten enough off? I would imagine that the glue is inside the fibers and somewhat resistant.

Mary, if you use warm water, a couple of thorough washings should pretty well eliminate the glue. Really no way to tell if all the glue has been washed out, unless the canvas feels tacky when damp-dry.

Thanks Richard, You are a life saver.

Some but not all RSG is rotted and smells bad - no matter the brand. It happens.

Fortunately, you do not need it anymore. Modern acrylic gesso can be applied directly to your canvas fibers - either cotton or linen. As long as you seal the fibers so the oil paint does not touch the fibers, you're good to go.

It is better to apply several thin coats of gesso than one thick one. Sand between coats.

Historical note: Some painters used RSG (water soluble) to seal the surface of the canvas - and then painted directly on top of the RSG. The color of the RSG and the canvas surface was used as the imprimatura.