Cennini on Painting: October 2008

Driers for Oil Paints

Lead driers dry the paint throughout the paint film at a constant rate. They do that by forming their own oxygen in the form of peroxides. All other driers are surface driers that rely on atmospheric oxygen to cure. They dry and form blisters of hard paint on the surface and soft paint inside. Japan drier is a generic term and can contain any of a number of metallic salts, usually there's some manganese in it. Manganese driers are brownish. In its concentrated form, cobalt drier is cobalt blue in color.

Siccatif de coutrai is supposedly a mix of lead and manganese driers. We've tried a number of recipes and none were very good, they darkened almost immediately, so it makes me think that the stuff that Bouguereau used was very different from anything written about. Lead napthante is clear.

Lead Naphthanate

Thus far, lead is the only drier that works from the inside out rather than the reverse. That's why most other driers can cause cracking.

Our lab guy demonstrated the proper proportions when he took the palette knife and mixed a small amount into each nut of paint (about the size of a walnut). It was no more than 5 drops gently stirred into each nut of paint. It improves handling slightly (all lead compounds do) and the paint is solid in the morning. AT that point you can scrape high parts (the Dutch Little Masters got that smooth finish by scraping and sanding between coats).

You can add Lead Naphthanate to anything that contains oil. It won't mean much to resins, so adding it to picture varnish won't do much. Walnut oil is lacking some elements that allow it to take full advantage of ead napthanate. In Stand Oil, one drop per tablespoon would be enough for most uses. Two or three drops will REALLY speed it up.

Cobalt Drier

Cobalt linoleate siccative is the proper name. A linoleate is a drying varnish made with linseed oil. A siccative is a substance that makes paint dry. The practice of cooking heavy metal salts into linseed oil is an old and honored practice. Cooking lead salts into linseed oil produces a drying oil that dries from the inside out. Cooking cobalt into linseed oil produce a drying oil that dries from the outside in. If you use thick paint, cobalt linoleate will dry with a skin on the outside (as most paint does), rather than all at once, as lead-bearing paints do. It's a deep, bright blue (cobalt blue) and tints all colors when used straight. It is usually diluted with mineral spirits to between 2% to 5%. It dries from the outside-in, essentially making a blister of paint. ...Cobalt drier dries quite brittle. It comes in two forms. In mineral spirits, it is deep blue and very difficult to use because of it's tinting strength. Cooked into linseed oil, it becomes brownish and much more tractable. That is then thinned with mineral spirits and called, cobalt linoleate siccative.

Manganese and cobalt driers are surface driers. They attract atmospheric oxygen to the surface of the paint and that's why the paint dries like a blister. Lead driers work in a very different manner. They produce internal peroxides, causing the paint to dry evenly throughout. The surfaces driers are faster than the lead driers. They also embrittle the paint film whereas lead strengthens it....The cobalt or manganese driers will accelerate polymerization on the surface, where it is in contact with the atmosphere. Lead napthanate produces internal peroxides that dry the paint film evenly throughout.

Olio Verde

The recipe we have calls for holding the oil at 275F for a week, under a vacuum. We start with Special Aged Linseed Oil, a copper salt and litharge. Over the week, the resulting oil has thickened considerably to an amber color that has a greenish tinge when put to the light or mixed with turpentine. It is WONDERFUL stuff, very fast drying, hard and flexible with a good gloss. It's also very thick and must be diluted.The recipe we are using dates back to somewhere around 1910, so it takes advantage of modern materials. The trick is low heat and vacuum over a long period. We begin with copper and litharge. Just remaining in contact over time will cause the oil to take up some lead and copper. Being heated (about 275F) for a week or so causes it to thicken and take up the oil. Sun-thickened oil is so variable in quality that I stay away from it. The good stuff is good. The bad stuff looks just like the good stuff.

Like stand oil, Olio Verde is made to exacting standards by professionals who know what they're doing. I thin it with two parts of turpentine for use in diluting colors for the underpainting. It's smooth and dries hard overnight.It is quite different from a true cooked oil like Black Oil.

Siccatif de Coutrai

Siccatif de Coutrai is a mix of lead and manganese. For Cennini, we have tried every published recipe for Siccatif de Coutrai (mostly blends of lead and manganese driers) and every one of them darkens badly. I wouldn't touch any of them with a barge pole.

Q: How does japan drier differ from cobalt drier?

RH: Basically, there's no standard recipe or method. One man's fish is another man's poisson. Some might be good, others might not be. It's a generic term (like Siccatif de Coutrai) whereas, 32% lead napthanate, 12% cobalt linoleate siccative leaves little doubt as to what to expect. You know that the lead is in a specific concentration and probably in a mineral spirits base. The other tells you that cobalt salts were cooked into linseed oil to make a linoleate. Japan drier gives no such clue. From what I have seen, most are made with manganese salts and whatever else happened to be kicking around (except lead). As I said previously, manganese makes paint brown, very brown.

Perhaps the most common misconception is to think of Alkyd resin as a drying additive. It's one of the many synthetic plastic resins made from petroleum products. It has a drier added and is often incompatible with oil paints (according to the manufacturers, it should not be mixed with oil as it can causes delamination and flaking paint layers).

Q:...the bottom of the bottle is covered with a white precipitate. Sugar of Lead? Just white lead from reaction between lead oxides and fatty acids?

RH: It's a form of lead acetate that precipitates out of solution when lead salts come in contact with some oils. Perfectly normal and nothing to be concerned about.

This doesn't really belong under this topic
but...Allowing the painting to dry face down is the best. In that way it fuses
in all directions rather than in the pull of gravity. Just place small blocks at
each corner and lay it face down with those blocks supporting the painting. BTW
this is a good way to dry paintings free from dust.
Q: This is something I've always been curious about. When people place a painting face down on blocks, do they not paint all the way to the edge or what? I mean, don't the blocks mess up the portion of the painting that rests on them?

RH: They don't come in that far. Make them with thin lattice. The actual blocks at the bottom are less than 1/2". You can also use pushpins in the corners.