MITRA Forum Question Details

First it is always a good idea to size. But obviously this is a choice left up to the artist. Directly relating to the earlier query posted here, we have actually been discussing the topic of "oil rotting canvas" recently and truth be told this topic remains one that requires further study. Such concerns can be traced back among artists for hundreds of years. Yet at the same time there are many, many examples of unsized paper (so made with cotton and/or linen fibers) that printmakers used as primary supports for oil-based prints (intaglio, lithography, etc.) that are hundreds of years old and in spectacular condition. HOWEVER there are certainly concerns regarding the chemistry involved with oil as it ages and degrades over time and the adverse affect that it can have on cellulosic fibers.

This excerpt taken from the Chemical Principles of Textile Conservation (pg 117) may be of some interest:

"When applied to a textile without a size, the liquid oil binding medium may penetrate into the fibers and impregnate the textile around the painted pattern. During network formation the oil film may shrink. The staining and shrinkage can damage both the appearance and structure of the underlying fabric; fibers can be damaged by the hardness and deterioration of the oil film. The presence of the peroxide crosslink in the film can result in chain scission of the carbon chains. The peroxide crosslink is a weak one, hence it readily undergoes ruptures caused by environmental heat or light. When the peroxide crosslink breaks the carbon to carbon bond located in the alpha position to the peroxide crosslink may also break. The resulting radicals may quickly undergo oxidation. […] The increasing acidity of the deteriorated oil layer may cause not only further fragmentation of the oil network but also hydrolyic breakdown of fibers, when it is in contact with cellulose, such as unsized paper, canvas, or barkcloth."

As for your image it does not appear to us that the areas designated with arrows are due to staining from the oil medium; however, it can be difficult to make these determinations from photographs alone. For the moment we still recommend applying a quality size and ground to your canvas before applying subsequent layers of oil paint.

To our knowledge there are no images out there floating around that demonstrate "oil rotting on canvas"...although some of our conservation colleagues that work with archaeological or ancient textiles may have some evidence/information that we are not privy to in the paintings world. But with fabrics of significant age it can be hard to pinpoint one factor and one factor alone that might have accelerated certain degradation processes....this is why we still feel it is a topic that is up for debate when it comes to traditional easel paintings.
As for the presence of fillers/pigments you will still have the same chemistry at play. Painting directly on top of an unsized and unprimed canvas will still involve some sinking of the oil medium directly into the overabsorbent canvas beneath. You would also risk creating an underbound paint layer to boot. But as to whether these reactions would happen to the same degree it is hard to say...it likely depends on the pigment-binder ratio and what type of pigments/fillers are present. Hope this is somewhat helpful!