It’s important to test the ph before you use a batch of soap – if it’s higher than ph10, you risk it being too harsh on your skin. Worse, if it’s too high (lye/ caustic soda is ph14) it indicates you’ve made a mistake and the caustic soda hasn’t processed properly – this will badly burn your skin. It’s a good idea is to cut the soap open and measure the ph in the middle of the soap, to make sure it is processed properly the whole way through the bar. Lye can sit in ‘pockets’ if it doesn’t mix. ph testing kits aren’t 100% accurate, so err on the side of caution if it tests high – leave your soap to cure longer.