Question: Compensation for evaporation

Terri wrote this comment (and Priya seconded it):  My nagging question is how do you account for the loss due to evaporation of ingredients during the heat and hold? Have separate pots with just water and/or other ingredients heating as well to add later? And I never checked, but is there a change in the weight for oils,butters after the heat/hold as well? It just doesn't seem like my 100g 'experiment' doesn't equal 100 in the end.

Great question! You will see evaporation in your water phase after heating and holding; it's unlikely you'll see a big change in your oil phase after heating and holding. I wrote a little bit about surface area and evaporation in this post, but let's get into more about compensating for the water loss.

If you are heating and holding your ingredients for 20 minutes at 70˚F, you will see some evaporation from your water phase. If you add the water to the oil phase without compensating for that loss, you'll have a lot less water in your lotion - you might think you're making a 70% lotion but are really making a 60% lotion. This isn't a huge problem in the sense that you will still get a stable lotion, but it's hard to replicate that wonderful product if you don't know how much was lost.

Here's what I do. I measure out my heated water phase into my heating vessel (usually a Pyrex jug, but sometimes my awesome Christmas beaker). Then I measure the jug and water phase together so I know how much they should weigh at the end. Write this number down (believe me, you think you'll remember it, but you won't when you get into playing with all kinds of other things to kill the heating and holding time!). Start your heating and holding. In the meantime, weigh out some distilled water and heat and hold that for the 20 minutes (I'll call this auxiliary water amount to make things easier). When you take your heated water phase away from the heat, just before you add the oil and water together, weigh the heated water phase container. You'll notice there's less water in there - so add the auxiliary water amount to make up the correct amount. Make sure the water you're about to add has the same temperature as the heated and held water and oil phases before adding it. Now add the water phase to the oil phase, mix, and enjoy your lotion!

As a note, your auxiliary water amount can be anything watery like aloe vera, hydrosols, and so on, but I generally use water.

I hope this answers your question!