Additives: clays, exfoliants and milk
Updated
Everything added after the oils and the lye, and each kind brings its own failure.
Four kinds of addition
Things that color, things that scratch, things that add sugars, and things that are there for the label.
All four change the batch, and the third changes it violently.
Clays
Kaolin, bentonite, French green, rhassoul.
They color gently, add slip, and are widely used as an anchor for fragrance.
Disperse in a little water or oil before adding, or they clump and streak. Dry clay stirred into a traced batch is a speckled bar.
And they accelerate trace, sometimes markedly, which matters if you were planning a swirl.
Exfoliants
Oatmeal, poppy seed, coffee grounds, pumice, seeds, salt.
Two rules. Not too sharp: anything with an edge scratches rather than exfoliates, and walnut shell in particular has a reputation for it.
Not too much: a heavily loaded bar crumbles and sheds in the shower.
Salt is its own product: a salt bar needs a very high coconut content and must be cut early, because it sets hard and fast and will shatter if left.
And remember what an exfoliant does to the label: describing it as exfoliating the skin is a cosmetic claim, while saying the bar contains oatmeal is a fact.
Milk, honey and sugars
The additions that cause the most trouble, and for one reason: sugars generate heat.
Milk, honey, beer, fruit purée and sugar all react with the lye and add heat on top of the reaction's own, which produces overheating, cracking, volcanoing and scorched, orange, ammonia-smelling soap.
The controls: freeze the milk and add lye slowly to the slush, keep everything cool, do not insulate, and consider refrigerating the mold.
These additions are also the ones most likely to be in the product name, which means the batch that is hardest to control is the one you have promised somebody.