Second Sun-thickened Oil Project

Sun-thickened linseed oil has been one of the staples used in traditional varnish making for a millennium or more. It takes patience and vigilance, and normally up to three months depending a latitude.  At my latitude the process can effectively only be performed in the dryer summer months.

As part of my ongoing period varnish experimentation, I’ve taken a couple of stabs at producing sun-thickened oil. The first experiment was limited to a funny-looking recycled glass jar, placed on a non-south facing window sill. The result wasn’t bad, all things considered. In about a year’s time the oil thickened to the consistency of warm honey. When heated for a test varnish mixture it darkened and polymerized much more quickly than I expected. This is a very good result, although the varnish was a little over-cooked in the end.

Lessons learned:

The oil needs to be in a wide, shallow tray for air and sun exposure. The oil must absorb oxygen with direct UV exposure from sunlight as a catalyst to partially polymerize. To that end it must also be stirred daily to prevent the formation of a skin.  Also, the most important lesson of all, most glass is opaque to ultraviolet radiation. All of these issues together make it ‘clear’ why it took a year.

This year’s experiment corrects the above issues and includes a truly Medieval period experiment. While most of the batches are in glass trays, to be covered with sheet glass only during inclement weather, one is in a lead try which will not be covered, will be allowed to form a thick crust and suffer all that nature subjects it to. This is the method described in a Medieval text on varnishes and artist’s materials. Hot filtering at the end of the season will be used to remove impurities. I expect I will have to filter out all manner of flora and fauna this fall. It’ll be like my own little tar pit.

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4 Comments

  1. Posted May 19, 2011 at 8:07 pm | Permalink

    Most of the oil is starting to thicken. I stir it once every day or two. The oil in the uncovered lead tray was a predictable disaster. Within a few days rain water displaced the oil which is now all over the plywood. I’ll try it again when the weather warms up.

  2. Mike
    Posted August 28, 2011 at 2:59 pm | Permalink

    Nice. Please post your progress now and then. I was thinking of making a batch myself.

  3. Max
    Posted September 20, 2011 at 8:41 pm | Permalink

    Chris, Do you have any leather costrels for sale?

  4. Posted September 20, 2011 at 10:23 pm | Permalink

    Hi Max,

    Thank you for your interest in my leatherwork. I do not have any vessels for sale at this time. I hope to re-open early next year. I appreciate any understanding you may have while we transition.

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