[Cob] Drying of linseed oil

[Quinn]

Finishing oil for cob--

A little bit about linseed oil:    (http://www.wwch.org/Technique/FinishesRL/OilFin.htm)

Originally, boiled linseed oil was the material of choice, and it was typically
mixed with equal parts of thinner, such as turpentine, and occasionally
small amounts of varnish and/or bee's wax were added. Linseed oil
is an oil extracted from flax, and the term 'boiled' is misleading but very
important. Boiled linseed oil is not actually boiled, but rather oxygenated,
a process that changes the polymer structure of the oil, which causes it to
eventually dry and harden. Raw linseed oil will never dry, and does not
make a good finishing material. Even boiled linseed oil dries quite slowly,
and the addition of varnish or driers is common these days to speed drying
and provide faster build of the finish. Tung oil is another commonly
available finishing material, though more often than not, off-the-shelf tung
oil finishes are not true tung oil at all, but rather a mixture of the materials
I've previously mentioned. Tung oil has become a somewhat generic term,
encompassing many varieties of oil finishes.

And about tung oil:    (http://www.sutherlandwelles.com/tungoil.htm)  

(of course, they're both talking about use on wood and this second site is hyping its product, but its interesting to compare them anyway.  Don't know the cost comparison, but I imagine tung oil is more expensive.  Has anyone tried it with cob?  I know its very hard, finished.)

Penetrating finishes formulated with linseed, soy or paraffin oils actually go into the wood and enhance its natural beauty, but these oils often dry incompletely and fail to form a hard and durable surface. Furthermore, they develop a gummy build-up when additional coats are applied. Linseed oil, the most commonly used penetrating finish, darkens and changes color with time and finally disintegrates. 

A Polymerized Tung Oil finish is hard yet flexible, waterproof and impervious to alcohol and many food acids.
As Tung Oil dries and cures, the molecules join together in a tight complex formation. This process is the secret to Tung Oil's effectiveness as a finish. The cross linking of the oil's molecules makes the surface waterproof and impervious to many chemicals. The bonding also gives flexibility to the surface, making it capable of withstanding wear and tear. 

Sutherland Welles Ltd.® uses a process called polymerization to intensify Tung Oil's natural cross- bonding tendency. Polymerization is essentially a cooking process that enhances the molecular structure of the oil and further improves the natural cross-bonding reaction of raw Tung Oil.  The polymerization process used by Sutherland Welles Ltd.® is very sophisticated and "cooks" the pure, raw Tung Oil to its "maximum thermal threshold." Controlling the "cooking" and the "cooling" is an expensive process that at its perfect point produces an oil that has maximum durability and a gorgeous sheen. If the process isn't well controlled the oil solidifies to the consistency of a rubber eraser and the entire batch is lost. The intricacy and therefore expense of the process is what leads most companies to formulate with modifiers. 

The Polymerized Tung Oil must be formulated with solvents to thin the oil to maximize its workability and penetration and driers to shorten the drying and curing time. Sutherland Welles Ltd.® has researched and implemented driers with the lowest toxicity available. We also use solvents that are "cleaner" reducing the environmental impact and minimizing a finisher's exposure to toxic fumes and chemicals. 

We at Sutherland Welles Ltd.® are proud of our commitment to improve product performance while reducing its environmental impact both to the earth and to wood finishers using our products.