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.

This entry was posted in Home, Leather Bottling. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.


  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.

Post a Comment

Your email is never published nor shared. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>