You've researched the safety of lye, tackled the task of using a soap calculator and made what appears to be a successful batch of CP soap...but is it successful? Testing the soap's pH is a critical part of soap making to ensure your soap is safe and within an acceptable pH range. Soap can range from a pH of 7 to 10 with most soap being 9 to 10. Soaps greater than 10 are considered lye-heavy and can irritate or burn the skin. Saponification, the chemical process in which lye and oils turn into soap, usually takes between 24-48 hours. After this time, the pH can be tested and it should be within the soap range of 7 to 10. Note that the pH can lower further during the 4-6 weeks of curing. Here are three ways to test a soap's pH.
Special pH strips give you an immediate indication of the pH. Wear gloves when handling soap until you have determined it is no longer caustic. Lather a small sample of cp soap from your batch in water, and rub the pH strip on the wet sudsy soap. Match the color on the strip to the color chart included with your pH strips. Read the pH next to the matching color on the color chart. We recommend using this method if you are a beginner cold process soap maker, you are creating a new soap recipe or you are a long-time soap maker who is selling your handmade soap.
Some soapmakers also do “tongue testing” or “zap testing”. With this method, you touch the finished bar of soap with your tongue. If the soap causes the tongue to tingle a bit (like touching the tongue to a battery), then there is still active lye in it and it should not yet be used on the skin. We recommend this method for more experienced soap makers who have previously determined the safety of their recipe and are just checking a new batch.
For this, you simply wash your hands with your soap. Soap with an acceptable pH will rinse clean and will leave your skin smooth and refreshed. If your lather instead feels slimy, difficult to rinse and leaves your hands with a reddened appearance (similar to doing dishes in hot water for too long) then your soap is most likely lye heavy and has not fully saponified. If your soap passes hand testing satisfactorily you may want to test further on more delicate skin such as the inside of your elbows before giving it the final approval. This method is also recommended for experienced soap makers who have previously determined the safety of their recipe.