CP/OP is the common soaper abbreviation for Cold Process/then Oven Processing the batch. It's actually a very simple procedure, but whats more important is understanding the Pros, Con's, and precautions of this method. How CP/OP works is that unlike regular Cold Processing where the soap is left to fully saponify over a long period of time, putting it in the oven with a low heat applied with the extra insulation of the oven itself, speeds the saponification process a long, rendering the lye null much faster.
The obvious and main pro is that unlike traditional cold process soap, it does not require a 6 week cure time. The soap is technically safe to use once it has completed it's full time in the oven, we still recommend at least a two week cure time to ensure harder, longer lasting bars, and to always PH/zap test before distributing! This makes CP/OP soap particularly appealing if you are on a deadline, running out of stock faster than you anticipated, or do a lot of craft shows and like to manufacture fresh to order.
There are however a few considerations. First, I don't recommend this process for new recipes, new fragrance oils, new essential oils, essentially anything you haven't worked with before. It's important to know how the particular recipe/blends behave; as some ingredients tend to super heat, such as honey, spice based fragrance oils or essential oils, white florals, and several others. It could result in a soap volcano going off in your oven, or worse (we know from experience, trust me).
Another consideration important to the success of the CP/OP batch is the flash point of your fragrance oil/essential oils. Since the soap will be exposed to high temperature longer (between 175-200 degrees for several hours) fragrance and essential oils with flash points lower than this tend to "burn off" resulting in soap that either doesn't smell at all or is very faint, or morphs into something unpleasant (burnt tire anyone?).
Lastly, consider your color scheme, if you use a bunch of lab colors, and even some oxides, with a complex design that is your signature or you're married to; this process may not be for that soap. The extended exposure to heat can cause colors to morph (we've had a few blues go florescent pink), fade, or bleed, and anything with vanilla, amber, or certain other resins will go dark, dark brown and bleed into everything else. Don't get too discouraged, if there is one thing I've learned in my eleven years of soaping is there are always exceptions and happy accidents!
Example of appearance of CP soap vs CP/OP : Renaissance (the lighter colored soap is regular cp the dark one is cp/op)
A few more things to go over before hopping to it:
1. If you have an older oven make sure you get an auxiliary oven thermometer so you have an accurate oven temperature, for this process you want to make sure it's between 175 and 200 degrees, lower the process won't complete, higher you'll either have a soap explosion or soap puffs.
2. You want to make sure your mold is heat safe for these temperatures. If you are using a wooden mold you want to be sure it's made with untreated lumber.
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