When soap seizes it’s just what it sounds like. It hardens suddenly. Most soap makers know the experience of having soap that is “moving fast”, it’s thickening up a little faster than you had expected, or would have liked. If it keeps thickening at a faster rate, it’s seized. I don’t know if there is a definition of the actual seizing point but I think it’s safe to say that it’s hardened too much to work with – to blend. Often it seizes after fragrance or essential oil has been added making it impossible to mix evenly or completely.
A lot of people seem to end up rebatching seized soap. I think it often might not be necessary to do this. And if I’m wrong.. you end up rebatching…so you really don’t have much to lose using my suggestions. (And I hate rebatching soap, it’s never up to par.)
I was a Hot Process soap maker for a long time before doing Cold Process. In HP soap making the soap goes through distinct phases of cooking. It gets very thick and stiff (like seized CP soap) then it softens up to a kind of mashed potato consistency and then goes on to get softer and have a vaseline-like appearance when it’s done. In HP soap you can add color and scent at this vaseline stage. My soaps (which are made mostly from olive and coconut oils though I’ve used others) may act a little differently than soaps made with other oils. When mine hit the vaseline stage they are too thick for a stick blender and take some work to mix by hand adding color and scent. You also have to work pretty fast because it’s cooling and hardening once it’s out of the oven (or crock pot). Note that overcooking soap does not seem to have any ill effect and it just stays at the vaseline stage so if you aren’t sure, cook it a little longer.
When the soap seizes it is just cooking and has gone to the next stage. Regardless of the temperature of the oils and the temperature of the lye before mixing them, soaping creates an exothermic reaction: heat is created. The soap cooks or saponifies – whether external heat is added or not. So, the way to fix it is to let it finish cooking!
Get the soap into a stainless steel pot or a crock pot (carefully…you still have active lye). Put it in the oven at 175 degrees or in the crock pot on warm or low (depending on your pot). Check it and give it a stir every 10 or 15 minutes until you get to the vaseline stage. This could take 20 minutes or an hour (maybe even longer). This too depends on the oils you’ve used.
When it gets to the vaseline stage it is as soft as it’s going to get. You should be able to stir it and mix in your scent and color. If it’s not give it another 10 minutes of cooking and if it’s still not you can add a little water, stir the best you can (or use a hand mixer if you have to. You don’t want to incorporate a lot of air but I’ve done this a number of times and not had a problem with air bubbles. ) and cook it a while longer to let it absorb more of the water you added. You can spoon/glop/pour it into your mold now. (A loaf or other simple mold will be best). What you end up with is a hot processed version of the same soap you started out to make.
If you try this, I’d like to know how it goes! Good luck.