On my Patreon feed, Sally asked: Under what circumstances can you tinker with a "finished" product? For example, a batch of shampoo isn't as foamy as I like. Can I melt some SCI and add it after the fact? Or can I make a large batch of lotion and then divide it and add separate fragrances afterwards? Are there categories of fixes than can be tried, or categories where just cut your losses and start over again?
This is a big and fantastic question, so I'll do my best to answer it in as much detail as I can. I will definitely be updating this post regularly as I think of more things!
Why wouldn't we want to add things after the fact?
1. It might mess up the emulsion. A lotion can only take x% oils before it falls apart. For instance, if you have a lotion that has a 24% oil phase and you've used 6% Polawax, adding even 1% more could make it fall apart.
Related post: Polawax and the 25% of the oil phase rule
2. It might mess up the emulsion if you have to heat it as a lot of emulsions can't take being heated again.
cetyl alcohol in it, and you wish to have it thicker, you could make a second 100 gram batch with more cetyl alcohol or another thickener in it, then blend the two together. Make sure you are compensating with more emulsifier if you're adding more fatty alcohol to a lotion!
3. It might mess up the product if you have to heat it. A lot of preservatives don't like the heat, so taking the product above 60˚C might result in the destruction of a preservative. Then you don't know how much is left, so you add more, and now you've used too much.
4. It might overload the preservative. If we have calculated the preservative just right - let's say you're using 0.5% liquid Germall Plus - and we add more stuff to it, it might not be enough and you'll get contamination.
5. There are quite a few categories of ingredients that don't do well at higher temperatures. Think about ingredients we add to the cool down phase as they can't handle heat - botanical extracts, some silicones, essential oils, alcohols, active ingredietnts - and you'll quickly see that there are so many things that could evaporate or become just plain awful if heated.
If you've ever had the misfortune of smelling Honeyquat when it's been heated, you'll know what I mean. Three words: Dead plastic fish...Ick!
Related post: How do you know into which phase to add an ingredient?
When would it be okay to add things after it's finished?
bubble bath, body wash, conditioner, and lotion to fragrance later. It might be that I like to change fragrances with the season or try new fragrance oils or give them as gifts with different scents.
Make sure you have accounted for this addition with your emulsifier. For instance, with Polawax, make sure you are using more emulsifier than you think you need - add 0.25% more for every 1% fragrance or essential oil you'll be adding.
Related post: Making large batches and scenting them later
Having said this, difference fragrances and essential oils can have an impact on the viscosity of your surfactant based products as well as gels, and can affect clarity, so be aware that a bubble bath that seems great with a citrus fragrance oil may turn to water when you add vanilla. This is why I never add my Crothix or other thickeners until after I've added my fragrance oil!
bubble bath - you could add things to it as long as you compensate with more preservative. If you've made a lovely body wash and you want to add 10% more surfactant, you might consider adding 0.05% more liquid Germall Plus, for instance. (Having said this, I use 0.5% liquid Germall Plus, the maximum amount, so I can add a titch more here and there if I want to increase foam and lather or include a new and exciting extract.)
I'm having a love affair with cold emulsifiers right now - things like RM-2051 (aka Emulthix) and Aristoflex AVC - and you could add things to these lotions after they're done as long as you are compensating with more preservatives and it can handle more of ingredient x. (For instance, don't go over 5% total oils with Aristoflex AVC.)
I think that's all I can think of at the moment! What are your thoughts on this topic? Share in the comments below!