A new soapmaker recently wrote:
“I recently relocated to the UK and the water here is much harder than what I was used to in the US. Do you know how this may affect cold process soap? Would you have any particular tip for dealing with this before I attempt my first batch? Not sure if buying distilled water is easy here... but I used tap water usually.”
That's a great question - and one that folks often wonder about.
One of the best bits of advice I've heard for whether tap water is good enough to use in soap making is “if your water is good enough (soft enough) to use your soap in (i.e. it lathers well enough, it rinses well enough)…then it should be fine to make your soap with.”
Hard water can cause soap “scum” to form –(more information about the chemistry of soap scum here), but you’re going to have that problem with any real bar of soap. (Less so with detergent bars.) Your water would have to be really hard to cause so much trouble as to not be able to use it in your soap recipes - and the rinsing problem comes with really soft water, not hard.
So, the quick answer is – you should be just fine with tap water.
I started out using distilled water in my batches soap. I read it in a book – and took it for the truth. I used distilled water in my soap for many years. Then, one day I ran out of distilled water...and had to make a batch of soap. I used my tap water…and the soap came out just fine. My water here in Dallas is considered "moderately hard" with "80-100 parts per million" on the hardness scale. For about 10 years now, I’ve been using just tap water, and haven’t had any problems.
Using distilled water is consistent. By using distilled water in your soap making, you know that you aren’t add anything that could possibly cause any problems with your batch of soap.
Soap made with distilled (or rain) water can be marketed as “pure.” This may be a significant difference to your clients.
Trace amounts of metals in the water may taint the quality of your soap.
It costs money. While not terribly expensive, using distilled water does add cost to your batch of soap; you may need to pass that cost along to customers.
It's one more ingredient to keep on hand. If you're trying to keep your soap supplies to a minimum, it may be an ingredient you can do without.
Do a few tests if you can. Make a batch with tap water and the exact same batch with distilled water. After curing for several weeks (I'd recommend 4-6), test them out. You may be surprised by how much or how little difference there is.