I've been rendering lard & poultry fat for years. I just read the Guardian article, and was amused (and a little frustrated) at the author's recommendation that the best way to render fat is on the stove, but with the caveat that you have to watch it carefully or else it might burn. However if you want to use the the slow cooker go ahead, but be aware that it will take longer! Well, duh! That's why it is called a Slow Cooker! I always do mine in the slow cooker. Yes, it takes longer, but I don't have to babysit it, it doesn't burn, it doesn't smell up the house (especially if you put the cooker outside). Put it on low, and leave it overnight, and in the morning: Lard!