EditDelete
Moderator Answer
(mkinsey)
Many artists enjoy the natural color and texture of unprimed linen, and choose to incorporate it as an element of the painting. Working on the reverse of primed linen is one approach, but doing so means losing some important advantages of primed fabric.
Sizing, the first treatment applied to a fabric painting support, stiffens the cloth (helping maintain a flat plane) and closes apertures in the weave network. Sizing also isolates fibers from direct contact with the oil paint vehicle, which can have destructive effects on unprotected canvas.
Priming provides texture, absorbency, and a neutral, uniform base color. Artist's primers are usually formulated with high solids to provide absorbency and hiding power, while reducing shrinkage and movement in the lower layers.
The reverse (unprimed) side of a canvas should still deliver some of the benefits of sizing in terms of maintaining a stiff, flat plane. The unisolated fibers could still react to moisture and humidity, however, and they will be directly exposed to contact with the oil paint vehicle. How much oil the fabric takes up will depend on the thickness of the paint and amount of oil painting medium used. If catalytic siccatives are also present, I think that could determine the speed and degree to which the oil affects the canvas. Unsized fabric will remain very receptive to dust and dirt, so any unpainted passages will be vulnerable in that regard, and probably hard to clean and maintain.
Untreated canvas tends to wick away a lot of binder from oil paint, leaving a waxy, low-shine finish that some artists enjoy. If too much vehicle is blotted up by the canvas, the paint film could be left weak with a chalky surface.
In my opinion, treating the fabric first with a synthetic sizing would probably be a sensible measure to protect the canvas and reduce excessive wicking of the paint vehicle while still revealing the natural canvas color and texture. The technique I believe would best manage the challenges of using the reverse side of primed linen would involve thin applications of paint that are minimally diluted, with no broad, continuous thick films.
(One last thought: While I don't know what materials Bacon specifically used, I expect the canvas sizing was probably animal glue rather than PVA.)
