One can think of this as a particular application of the treatment of the flexibility/inflexibility rule given above. It is apparent to anyone using a variety of oil paints that, even in identical conditions, drying speeds will vary greatly between them. Burnt Umber can be touch dry by the end of a summer’s day, whilst Alizarin Crimson, unmixed, can take about a week.
Michael Harding, unlike some other paint makers, does not attempt to equalize the drying speeds of all the paints in the range by adding to them siccatives or retardants which will ultimately have an adverse effect on their appearance. Artists generally appreciate getting to know the idiosynchrasies of their colours, and do not wish to think of them as some single homogenous stuff which just happens to come in different colours.
Drying speeds can be correlated with different families of pigments: the earths are generally the fastest, followed by most of the inorganic metal compounds, with the organics generally, but not always, being the slowest. These speeds are also approximately related to the oil content of the paint, i.e. the amount of oil needed to mix the dry pigment into a useable paint. As a rule of thumb, those paints with high oil content will be the slower. But there are exceptions to this, notably Red 48 ( Scarlet Lake Deep) which is a high oil content organic, but dries very rapidly. As you may have noticed Michael prints a drying speed rating on all his labels to avoid misapprehensions, (very fast- fast- average- slow- very slow).
The commonsense to which this advice refers is applicable when using conspicuously slow drying paints, on their own and without much thinner, and then overpainting them with layers of much faster drying ones. If the underlayer is still only touch dry, and therefore still highly absorbent, then the result might be that it rapidly robs the overpainting of its binder, causing the latter to mat out or “sink”. In the longer term there is a risk that the underlayers will carry on moving long after the top layers will have hardened and become less flexible, causing the latter to crack.
Oil paint is a fairly robust material, and it does not do to overstate these risks, otherwise painters would be anxiously calibrating the drying rate of every touch. But at an extreme there is the historical example of the use of Bitumen, much favoured as an underpainting brown from the18th-19th centuries, but unfortunately a substance which never properly dried and which in fact became liquid again in high temperatures. Many works from this epoch have developed wide surface cracks, or even have slid away from their canvases in a buckling sheet. Thankfully, no paint as mobile can be found in the modern range of oil paints!
There are three specific points which may be useful advice:
- Remember that slow drying paints can be in effect accelerated by mixing them with faster ones, and that those paints which dry to a tough paint film will strengthen those which do not.
- When using black or white paints largely pure, remember that drying speeds to some extent suggest an order of application. It might not be advisable to make heavy use of Ivory Black over the slower drying Lamp Black, nor to paint solid sheets of Lead White over layers of pure Zinc or Titanium White.
- Those paints where the colour is supplied not by a pigment powder but by a liquid, called Lake colours, can sometimes be slow driers. In a modern range of paints, lakes are invariably organic, i.e. hydrocarbon-derived, and they have the strong tinting power typical of many organic dyes. (One the strongest, Phthalocyanine Blue Lake, exceptionally, dries fairly fast). But the slower drying ones may not be the wisest choice for an underpainting that will be worked upon rapidly.
Some years ago there were reports of a slightly mysterious tendency of the organic lakes to “strike through” or “bleed”, that is, to very gradually overpower other colours in a mix, or over a longer period of time, to leech into an overpainted layer and stain it. Since there do not seem to be present-day accounts of these phenomena, we suspect that they resulted from manufacturers’ inability to adequately stabilize the first generation of the organic lakes used in artists’ paints.