Two pots of chicken fat, two of lard, and one of lamb tallow, which turned out to have lard in the bottom as well.
As you can see from the picture, whilst this stuff keeps fairly well, it's not perfect and there are little bits of mould that needed scraping out before I used this. Some of this has probably been in the fridge for well over a month.
I looked up the sap values* and found that for these three fats the values were all the same, which meant I could mix them and weigh the whole lot together, reducing the inaccuracy of weighing lots of small amounts. Having weighed that (pretty much exactly 1 lb in total), I added a bit of hemp oil (2 oz) that I still had in the kitchen, untouched since last time I made soap.
I then turned to a couple of online calculators to work out the final details of the recipe. This one has a neat little adjustment where you enter the size of the mould you're using and it resizes the recipe to fit. Using these, I added a bit of sunflower oil (1 oz) and tweaked the superfatting** adjustment to 7% so that I'd get a nice round number for the quantity of lye required (2.5 oz). Just for completeness, the amount of water in this recipe was 5.8 (a slightly generous 5 3/4) oz.
Having settled on the recipe, I put all the fats and oils in a big bowl*** and put that in the oven, which was still hot from bread-making, to melt the meat fats. While that was melting, I weighed the water into a plastic jug, carefully sprinkled the required weight of caustic soda into that, then took it outside so it could give off noxious fumes into the atmosphere rather than my kitchen.
When the fats were melted, I took them out of the oven and scooped off the small amount of scum that had floated to the top. Hmm, that could be a drawback of using saved fats. I may not have got quite all of the bits out, but still, I'd have plenty of time to fish them out while waiting for the mixture to trace.
Being rather less nervous about the whole process the second time around, I may not have waited very long for the two liquids to cool down before mixing them together. I did, however, follow John Seymour's advice and added the lye very slowly to the fats, stirring as I did so. The fats started to change colour immediately.
What amazed me was how quickly this reached the trace**** point. I didn't time it, but it seemed barely five minutes before I saw the first signs of it, and not more than twenty at most, before it was definitely tracing. I didn't even use a blender - I was stirring with a good old-fashioned spoon. Forget your fancy Castile soaps, this is the way forward!
Somewhat less than 24 hours later (i.e. this morning), I turned the soap out of the mould and cut it into blocks.
It was very soft when I turned it out, but it's hardening (and getting paler) with each hour. Just in case you were wondering about the smell (and I suspect you were), it smells lovely! Not at all like meat, just very soapy. Now, does anyone know how long lard soap takes to cure?
* Saponification values, that tell you how much lye to use for a given amount of fat or oil.
** The amount by which you adjust the quantity of lye in the recipe, to be on the safe side, i.e. erring on the side of a sloppy mess rather than something that'll eat through your skin.
*** Last time I used a bowl that was only just big enough to hold the oils, which made it difficult to mix without sloshing over the edge. I learnt from that mistake and used a very big bowl this time.
**** When moving a spoon through the mixture leaves a 'trace' behind in the surface. This is the sign that you can stop stirring and pour the soap into a mould.