Increased environmental awareness and the provisions of the Solvent Ordinance are currently leading to a renaissance of linseed oil paint without solvents for painting wood. Linum Usitatissimum, the very useful linseed, is one of the oldest cultivated plants known to mankind. Around 4000 years ago, the flax plant arrived in Central Europe from the temperate regions of Africa and Asia. Initially, its fibers provided the raw material for canvas and clothing.
From the 15th to the middle of the 19th century, painters and craftsmen used linseed oil to produce linseed oil paint. But then paint factories increasingly took over the production of (linseed) oil paints. As a result, the knowledge of handcrafted production was almost lost. Only small paint manufacturers from Sweden, Denmark and Austria never completely abandoned their traditional production methods. These are currently the main suppliers for paint dealers and processors of linseed oil paints, even though there are more and more regional suppliers of raw linseed oil in Germany.
Johannes Mosler is a carpenter and specialized in the restoration and conservation of historic windows 15 years ago. He has the most extensive archive of documented window restorations with linseed oil paint without solvents in the German-speaking world.
He reports on his experiences not only as a lecturer at training centers for monument conservation in Germany and abroad, but also at the denkmal trade fair in Leipzig. There he will be giving a lecture on the subject of this article.
Linseed oil and linseed oil paints – an overview
Raw, cold-pressed linseed oil
Linseed cake and cold-pressed linseed oil are produced by gently pressing linseed in a screw press. The last residues in the linseed oil settle during storage. The oil is ready for use after one year.
Due to its polyunsaturated fatty acids, linseed oil is a drying oil. Oxygen absorption or oxidation produces the resistant linoxin, initially on the surface. This increases the volume by up to 15 %.
Sun-oxidized linseed oil
Raw linseed oil is exposed to the sun in shallow trays until a honey-like, viscous consistency is achieved. Stirring daily prevents a skin from forming. The resulting sun-oxidized linseed oil serves as a binding agent in the final coat, just like linseed oil stand oil.
Linseed oil – stand oil
In the past, linseed oil was filled to the brim in containers and left to stand for a long time in the absence of air. “This allowed mucilage, impurities and other deposits to settle to the bottom, and the material, which had thus purified itself to a certain extent, could be skimmed off or extracted from the top. “1
Today, linseed oil stand oil is understood to be linseed oil heated to over 230° C in the absence of oxygen, which thickens slightly as a result of this process. According to DIN 55931, it must not contain any additional drying agents.
Linseed oil varnish / boiled linseed oil
Numerous recipes for “varnish” have been handed down, all with the aim of making linseed oil easier to process for painting and varnishing and improving its drying by pre-oxidation and the addition of natural resins. In the 18th century, the term “varnish” was used for linseed oil that had only been boiled.
Other methods of varnish production include
– Oxygenation by blowing the oil
– Combinations of boiling and blowing
– Boiling with the addition of siccatives such as manganese or cobalt
– Adding siccatives in a cold process
Drying agents / siccatives
Commercially available linseed oil paints and varnishes contain small additions of manganese or cobalt as drying agents. Paints without additional drying agents can be produced with pigments such as white lead, zinc white, micaceous iron oxide and magnetite, which have drying agents in their own right.
The full article on the preservation, conservation and care of wood with linseed oil and linseed oil paints can be found in RESTAURO 07/2012. In this issue, Lena Lang also reports on the conservation of a museum ship and Roswitha Friedelt presents a practical aid for restoration: a silicone plaster.
Note: 1 Kurt Wehlte, Werkstoffe und Technik der Malerei, Christophorus Verlag, Freiburg, 2009, p. 224.