Excellent stuff Hamish.
When I was over in Karmoy they were making tar on a semi industrial scale to tar the New Longhouse.
Your scaled down version is a lot more manageable.
Hamish. I've made some leather treatment which (I think) is 50% tallow/25 beeswax and 25 lard. Dampen the leather and apply and rub in.
Makes really nice hand cream too!!! Ping me your address and I'llpop one in the post if you want.
stinky stuff isn't it?
I've made a few litres of birch oil this way over the years (normally about half a bean can at a time). I use the nice steel biscuit/sweety tins too. When reducing it to make tar or thicker oil, I've found two things: don't rush it by rapid boiling as you end up with lots of crunchy bits in the oil or a bean tin full of flames; secondly it take hours to reduce it.
The last batch I made (was a couple of years ago, so may've forgotten something) I reduced it down on the back of my woodburner over night. It was a gently simmer, so didn't break down whatever is in there to go crunchy and the liquid went from water thickness to more like syrup or treacle. I was using it as a resin to glue historical knives together, so mixed it with beeswax and charcoal dust.
I have a fair bit of birch and have considered getting a kiln together to produce it properly, but I don't really have a good (ie economical/financial reason) to make lots of tar or oil.