I have a tupperware of old cooking oil in the fridge that I've been debating throwing away - but I would rather re-use it than just toss it. Can I render it and make it into jar candles? It's basically just multiple layers of "waste" cooking oil with impurities from whatever I was making at the time. Could I render it like tallow/lard and just strain out the solids?
Sure you can. Bring it up to heat and let it bubble and simmer for a bit, skim off any scum that floats to the top. Then break out a good sized funnel and some cheesecloth or paper towels, line the funnel, and slowly strain the still-warm oil into a holding container. You can then use this oil for survival candles. Just remember to keep a decent supply of shallow metal or glass containers and wicking material onhand. Here's a link to an Instructable page talking about the process and how to best adjust the "candle" so that you get no soot/smoke. Best of luck!