Gilding With Oil Painting

Traditional burnished gold could only be accomplished with water gilding. This means that it needed to be applied over an animal glue containing ground and usually over an animal glue containing colored bole. This means that the ground must be true animal glue containing gesso or animal glue-chalk. I understand that there are other methods that achieve mirror-like gold effects. Kölner Insta-Clay is one such product. I have never used it so I cannot comment on its efficacy.
The reason why you cannot burnish oil gilding is that you are actually burnishing the ground and bole, not the gold.
When oil gilding or polymer gilding it is usually best to apply some sort of sealer to cut the absorbency. This prevents having the gilding fail where too much of the size was absorbed into the object. I know that shellac was often used for this purpose but I would assume that any was approved for permanent work, dried insoluble, and could be applied smoothly and without brushstrokes would work well. Having it in a spray can would be helpful as well.

Sorry to confuse you. There is no “unmodified” ground that is appropriate for burnished gold (meaning water gilding) and oil painting. The water gilding requires a chalk or gypsum/animal glue ground with or without a bole/animal glue layer) to allow for burnishing. These grounds are very brittle and need to be applied to panels and not canvas. Additionally, such a ground is excessively absorbent for oil painting. Traditional gesso and chalk glue grounds do contain animal glue and are, therefore, reactive to changes in humidity. They can be very “archival” as evidenced by paintings and objects in museums containing these materials that are 700 and more years old. More to the point, unlike general easel painting where we have numerous grounds which are less sensitive to the eviroment
Sorry to confuse you. There is no “unmodified” ground that is appropriate for burnished gold (meaning water gilding) and oil painting. The water gilding requires a chalk or gypsum/animal glue ground with or without a bole/animal glue layer) to allow for burnishing. These grounds are very brittle and need to be applied to panels and not canvas. Additionally, such a ground is excessively absorbent for oil painting.
Traditional gesso and chalk glue grounds do contain animal glue and are, therefore, reactive to changes in humidity. They can, however, be very “archival” (I dislike that word but understand what you mean) as evidenced by paintings and objects in museums containing these materials that are 700 and more years old. More to the point, unlike general easel painting where we have numerous preferable grounds which are less sensitive to the environment, water gilding really requires an animal glue binder. In this instance, the oneness is on storing the work in the proper environment.
This problem is easily surmounted in the following manner. Prepare your whole panel with a chalk or gypsum/animal glue ground. If desired, coat all areas to be gilded with bole/animal glue. Apply gold and burnish. Then coat all areas to be painted in oil with a layer(s) to diminish the absorbency of the ground. In the Renaissance, this was done by applying a few coats of animal glue and then an imprimatura (basically, a fatty oil paint layer containing white). This is still a viable method, however, today this could be accomplished by cutting the absorbency with an acrylic dispersion medium, shellac, the judicious application of a drying oil (perhaps with a bit of catalytic drier added), or perhaps best, with an alkyd paint or medium layer. One need to be cautious to make sure that this layer only cuts the absorbency and does not create a slick surface that may promote paint delamination.   
As I wrote previously, there are a few systems out there for creating the appearance of burnished gold that do not rely on animal glue grounds. I am not experienced with these materials, but will forward this query to a moderator who may have more knowledge about these systems.

That ground is a very lean polymer dispersion ground. Geroge knows his stuff and it is probably appropriate for tempera. I am not sure that I would recommend it for burnished gilding, though. It would dry water insoluble meaning that it cannot be reactivated during water gilding, and would not take a burnish. Your bole would be burnin\shable as long as it is bound in animal glue, but this layer is usually very thin and the burnish would be less than normal. I will ask George to comment here, as perhaps there is something in the formulation that I am missing that makes it appropriate for this purpose.​

Rublev Colours Tempera Ground is based on a vinyl acetate ethylene (VAE) dispersion that allows us to formulate it with a very high pigment volume concentration (PVC), much higher than is possible with acrylic based grounds. For this reason it has much better absorbency and sandability properties than acrylic grounds and gets as close as possible to the PVC of traditional gesso grounds, which is the type of absorbency desired for gilding. We have tested gilding on this ground that can be applied to either wood or ACM. However, to achieve “bright gold” or very high gloss burnished gilding requires very smooth surfaces as Brian has pointed out. This is possible by applying multiple layers of bole (red clay) in animal collagen glue. The bole can be sanded and then burnished to achieve the smooth surface required and the animal collagen glue in the bole can be activated to adhere the gold leaf. Although some gilders apply gold leaf by breathing on the bole, we have also found that applying a “gilding liquor” allows better activation of the animal collagen glue.

Rublev Colours Tempera Ground is a very absorbent ground and hence is excessively absorbent for oil paint (and hence why we named it “Tempera Ground” which requires very absorbent surfaces). This can be easily corrected by applying a toning layer or an imprimatura of oil paint. We also recommend adding a small amount of bodied oil to this paint to enhance the “hold out” properties of the oil paint.

When I assign a gilding project to my classes, they generally follow this procedure.
Sand panel with 220 grit paper
Dissolve the animal glue in a double boiler. Apply a thin application of animal glue size to all sides of the panel (they work on wooden panels as opposed to ply or hardboard). You may only want to only size one side but this may cause warping unless the panel is braced on the back. If you size both sides, you may want to paint the back after finishing the front to isolate the glue from the environment.
Let dry and apply a second layer. Let dry.
Warm the animal glue in the top of a double boiler. Remove top from the bottom water-containing pan. Make the ground in the top. Gently stir. Apply as many layers of chalk/gypsum and animal glue ground to the panel as can be accomplished in one long session. Breaking this up into multiple days promotes pin holes in the ground. When the ground become too cool, and gels, warm the bottom water-containing pan in isolation from the ground-containing pan. After the water is hot, remove it from the heat source and place the top, ground-containing pan on the bottom of the double boiler. Never heat the ground directly or indirectly ON the heat source. It should only be made fluid by putting it on the pre-warmed water. Over heating the animal glue containing ground is the surest way to promote pin holes in the ground.
Let dry. Sand smooth.
Mix bole with animal glue. Thinly and smoothly apply the bole to places where you intend to gild.  Apply a couple of fine layers until it is opaque. Let dry.
Polish the bole.
Water gild.
Burnish when the ground and bole are ready.
Apply an imprimatura, etc. layer to regions to be oil painted.
Allow to dry.
Paint away                                                                 The brilliance of burnished gilding is diminished by a varnish. If that matters to you, I would only varnish the oil painted portions.You could paint on the areas of the panel that only have animal skin glue as long as you take the color of the panel into consideration.                                                                                                                                                                                      Oil paint does not adhere to glass well at all. Reverse painted on glass paintings are the bane of all painting conservators. If one was dead set on doing this, I would suggest sand blasting the glass first so that there is some mechanical tooth but this may still be problematic.

I agree fully with both Brian and George’s statements on Gilding and Panel preparation, and gilding materials. As to the questions #2 Why can’t you burnish oil gilding? The answer is about materials used, the burnish or shine comes from the direct contact of burnisher/agate stone with the Rabbit Skin Glue, as that is the surface material that is actually taking the “shine”. Do not allow the burnisher/agate to touch the oil gold or oil gilding, for if it does, the gold will come off, and the surface could brake down.  
​#3 Gilding a Ram Skull; first there are questions; is this a found object or store bought, as there may be chemicals used to remove sinew etc. that can effect the surface preparations. One can test as “Bone” is fairly porous and a simple test of brushing rabbit skin glue directly on a small area, allow to dry 2 hours, and with a sharp knife scrape the edge of the rabbit skin glue, if it should peal off switch to Oil Size for your size/mordant, if it adheres well, cover the entire skull inside and out with 2 coats of warm RSG and allow to dry. You then can treat the skull like a frame or panel and begin using “Gilder’s Liquor” warmed and brush small areas and gild directly. Allow a minimum of 3 to 4 hours after gilding including removing any skewing’s and applying and regilding misses before “Burnishing”. Please remember that using an agate stone requires a “practiced hand” not too soft or to hard, bring the “shine” up consistently and when completed, seal with a “Clear Overcoat Varnish” by Ronan “Topcoat Clear” gloss which is a water clear non yellowing acrylic enamel for use over gilded surfaces.  Martin Kotler