Finishes for Wood-Carvings
1. Why does a woodcarving require a finish? Why not leave it “in the raw”?
- To avoid ingress of dirt and grime into the wood.
- To reduce changes in moisture content in the wood, hence minimising swelling and shrinkage. This is especially important for exterior work, exposed to sun and rain.
- To enhance surface appearance. Here two stages may be considered – the application of stain, followed by a surface treatment.
2. The Structure of Wood
Wood always contains some water, and its moisture content will depend on the moisture content of the surrounding air, as well as its ability to absorb or lose water according to its surroundings.
A simple way of imagining wood structure is to think of a bundle of drinking straws, so wood takes up water and dries out much more readily from its ends than from its sides.
This is the reason why it is good practise to seal the ends of a piece of green timber before seasoning, as this avoids much faster drying at the ends, with faster shrinkage, and consequent splitting.
Also, when staining a wood-carving, it must be remembered that end-grain will take up much more stain than side-grain, creating a patchy effect. So end-grain areas may need to be partially sealed before stain application, or a thinned-down stain applied to these areas.
3. Staining Wood
The most suitable stains to use for carved work are dyes, in which the colour is dissolved in a solvent. The solution is applied, the colorant soaks into the wood, then the solvent dries out, leaving the stain on the wood. Stains can be thinned by use of the appropriate solvent where a lighter effect is required.
Traditional dyes made from coffee, tea, berries, walnut husks, logwood, cochineal or dragon’s blood are water-based, and tend to raise the grain during application. But now chemical-based dyes, usually dissolved in a white-spirit base are readily available (eg. Colron and Rustins products), which do not have this disadvantage.
Whatever the stain used, plenty of time must be allowed for the solvent to evaporate completely before applying a further finish, otherwise a sticky non-drying coating may result.
4. Oil Finishes
Boiled linseed oil and other blends have become popular with wood-carvers for their ease of use and the way they enrich the grain in the wood . Matt, satin or gloss finish can be produced, according to the number of applications.
Linseed oil, (from the seeds of flax plant) and a few other natural oils differ chemically from the mineral oils used for lubrication, in that they cure, on exposure to air, forming a solid skin. If applied too thickly, this skin will prevent the oil beneath from further exposure to air, so that a sticky, soft, wrinkly finish is produced. So this is why it is necessary to wipe XS oil off shortly after application. The oil must be in the wood, not on it.
Raw linseed oil takes weeks to cure, but boiled linseed oil has driers added, usually salts of cobalt, manganese or zinc, which speed up the curing process to a day or so. So this is the type to use.
Linseed oil finish is easily penetrated by water and water vapour, and marks easily when wetted, so is not protective to an out-door carving.
Tung oil (extracted from nuts of Tung tree) has been used for centuries in China, and does offer fair water-resistance after 5-6 coats. It roughens the wood, requiring sanding between coats, cures slowly, and tends to show white in pores and cracks. It is less yellow than linseed oil, so could be better on blonde woods. Seldom used as a finish in its own right, but often included as an ingredient in high quality varnish.