[FONT=Verdana, Helvetica, Arial]I received the following question from Ken Kaiser:

In the past of I have used both amber-based mediums and Winsor & Newton Liquin but have slowly been switching over to stand oil-based mediums for glazing. To improve the drying time of a mixture of odorless mineral spirits (OMS) and stand oil (Natural Pigments Vacuum-Bodied Heavy Viscosity Linseed Oil) of equal amounts, I added the recommended number drops of the Natural Pigments Cobalt System Drier based on the quantity of the stand oil. It appears to definitely improve the drying speed on a thin test layer on a canvas sheet but I was surprised by the mix the next day inside the small, closed, glass mixing jar (the ones with the closing compressed hinged top available at craft stores). The medium was extremely viscous as if the drier dried-out the OMS and more! I would say it was more viscous than the original undiluted stand oil. Is this common? Should I add more OMS? I like to have the mediums premade and ready to use. Maybe this is not possible with the use of OMS and driers. I have other mixes that I am testing, such as a Natural Pigments Epoxide Oil-OMS-stand oil 1:1:1 ratio that seems to be drying fine without this issue in another jar of the same type. Any comments or recommendations would be appreciated.

I just took another look at the very viscous stand oil medium again, somewhat perplexed and still bugging me. I decided to open the container and stir it with a metal tool and then realized the film on top of the medium was giving the appearance of very high viscosity. This film was keeping the medium from shifting (much) when the jar was tilted. After I broke the film and stirred it around, it seemed thick but much more reasonable. I did not see the film floating around in the jar after stirring. Do you believe this film or residue is still present and could interfere with obtaining a clean glaze. I am glad I investigated this further before you spent time on it.

My answer to Ken's question is as follows:

The Cobalt System Drier was primarily formulated for addition to oil paint in situ or immediately before using it. The drier may cause an increase in viscosity of the medium, but more likely it will cause a skin to form on the top layer of the medium. To prevent skinning, you can add a few drops of clove oil. Eugenol, the main ingredient in clove oil, acts as an antioxidant and temporarily prevents the medium from absorbing oxygen in the atmosphere the container.
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