Wednesday, May 28, 2008

Gumming the Inside

With the ribs and sheathing all ready, the next step involved gumming the inside seams of the canoe with pine gum. The kit had provided some commercially prepared pine rosin (without dirt & debris) that could be melted and tempered directly without the need to strain out any impurities. Sources write about different gums (like Spruce, Tamarack, but never Balsam gum) being used in bark canoe building. The gum itself is too brittle if simply melted and applied so it must be tempered with some source of fat. Traditionally this involved highly saturated bear fat. Given that wasn't an option, the urban equivalent used was commercial Lard. After picking some up at the local grocer, I set up for the event.

Given the potential flammability of pine gum and the fact that it really ruins cookware, I prepared it outdoors with an extinguisher at hand and used some cans destined for the recycle bin. The heat source was a Magic Heat stove kit.

The Setup outside; Melting the rosin; Adding in Lard

Books mention that enough fat must be added to make the gum pliable, but not too much to make it runny or greasy. This means adding a small amount of fat (I used half tsp at a time) and evaluating the gum. One way is to stir with stick and monitor the strands of gum that drip off. A perfect mixture gives off a strand that can bend without cracking.

A more visual method that I preferred involved dipping a sample of test bark in the heated pitch, then quickly dipping it into cold water. The bark sample is then bent back on itself and the gum evaluated for pliability. If there is insufficient fat, it'll crack and if there's too much fat, it'll be greasy and drip off. My own batch needed just 2 tsp of lard and a few tests to get it right.

Dipping test bark in pitch; Dipping in Cold Water; Testing for cracking

Gumming the inside also meant sealing problem areas on the bark. In particular, there was one significant branch hole on the port side as well as minor holes on the bottom. To seal these, I gummed the area and placed scrap pieces of bark over the holes, and then applied more pitch over these to glue them in place. Made for a messy interior but they'll be covered with sheathing soon anyway.

The portside knothole; Scrap bark pieces for the holes; Sealed up

The largest use of the gum was to seal the lap stiches from gunwale to gunwale. The pics below show one end and one side of gore stitches sealed.

Sealed lap stitches; Sealed gore stitches

I ended up using about half my supply of pine rosin so far, which means I should have more than enough to seal the outside and stems. To make the gumming look neater on the outer hull, I'll probably use some masking tape on either side of the stitches when I get to that stage.

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