A side note...Crothix is very like PEG-150 distearate used to thicken as well, but it is a complex ester, whereas PEG-150 distearate is not a complex ester. It's not a big difference, and I understand you can substitute one for the other. I've never used PEG-150 distearate, so I can't confirm this, but they do appear to be interchangeable. So feel free to experiment using PEg-150 distearate in place of Crothix and let me know what happens!
As a bonus, liquid Crothix contains PEG-6 caprylic/capric glycerides (coconut based), which offer superfatting and emollient properties.
So to sum it up - Crothix is a thickener, irritant mitigator, foam stabilizer, and emollient for surfactant systems. (Feel free to insert your own jokes about irritation mitigation here...I have a ton of them referring to my neighbours.)
How to use it...I generally use Crothix at 1 to 2% surfactant mixture. You can use it up to 8%, but this will result in goo, so I don't suggest it. (Although you can always make a jar of bubble slime in green for your kids! They love it!) Always add Crothix when your mixture has been fragranced, preserved, and everything else and when it has completely cooled. I mean really cool. I mean the next day kind of cool. You will never know how thick it is going to be until it is completely cooled (I think I've made the point).
Let's say I want to make a body wash with Jewelled Citrus in it. (And I've apparently illustrated this little story with a green body wash in Pearberry scent!) I know Jewelled Citrus will thicken the mixture at first but will eventually thin out to be only slightly thicker than water. I make the body wash with all the ingredients - EXCEPT CROTHIX - then let it sit overnight in a well covered Pyrex jug. Do not bottle it yet! The next morning I check the viscosity, then add 1% Crothix. Check. Is it okay? Usually it is, but sometimes I might have to add up to 0.5% more and perhaps another 0.5% more, but never ever more than 2% (my personal preference).
For more information on the effects of fragrances on viscosity of surfactant systems, please check out the post on fragrances and viscosity from March 7th and fragrances and clarity from March 6th.
Yes, you can get Crothix pastilles that are a tiny bit cheaper than the liquid, but this is just a bad idea. I can make exactly the same body wash recipe five times in a row with the same fragrance oil and 2% Crothix might result in a thin liquid or body wash Jell-O! Using the pastilles means you have to add them at the beginning and heat your surfactant mixture then wait for it to cool completely before finding out if it is thick enough. And you can't really go back and heat it up without compromising the preservative system. And it doesn't contain the PEG-6, so you're lacking an emollient there. So I always suggest you pay the extra $1 or so for the liquid!
Please visit these posts for recipes including Crothix!