thickening liquid soap

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many people have had trouble getting their liquid soap to thicken. according to ms. failor's "bible" on liquid soap, the addition of borax, (which also works as a neutralizer), will thicken the soap, but there are specific criteria needed to evoke this response.
•  high percentage of soft oils
a high percentage of the oils used to make the soap should be "soft" oils. the higher the percentage, the more pronounced the thickening effect.
•  minimum quantity of coconut oil
coconut oil is essential to produce a transparent soap, but this must be kept to a minimum. a soap made with all or mostly coconut oil (or other similarly hard oils) won't thicken. usually, you wouldn't use more than 30% because of it's drying effect, but there are sitituations where you might want to use more, (e.g. extremely oily skin, hard or salt water).
•  concentration of the dilution
the thickening depends on the concentration of the soap. the higher the concentration, the thicker the soap. in the past, i've made soap high in soft oils, and diluted for a high concentration, however, when adding the borax, the soap thickened only ever so slightly, remaining much thinner than desired. after "cooking" the dilution, (process described below), the increased concentration allowed for increased thickening.
•  no excess fatty acids
if your soap has unsaponified fatty acids, it won't thicken. failor's method of taking a -10% discount is to insure (for the sake of clarity) that there are no excess fatty acids in the soap. if, however, you want your soap to have a slight superfat, and instead, you take a regular lye discount (2% to 3%), the excess fatty acids will prevent the thickening response to the borax. in this instance, since the borax doesn't thicken, and it's not needed for neutralization, it can be omitted completely ...and you'll have to resort to some other thickening method.

if your soap meets all of the above criteria, but after adding the borax, it still doesn't thicken enough, it could simply mean that it's just not quite concentrated enough. this can be remedied by "cooking" the dilution essence, removing some of the water. we're only talking about a small amount.
note: the process described below is for small batches. it's not practical for larger batches ...3 pounds of soap paste would dilute to approximately a gallon of diluted soap. it would be impractical to bring that much soap to a rolling boil ...several times ...waiting for it to cool between heatings.
...when dealing with a large batch, all of which is to be diluted, i use this process to test samples to determine what the appropriate dilution should be, so that when the borax is added [to the whole batch], it will produce the desired viscosity.
...or if the whole batch has been diluted, but after adding the borax, it isn't thick enough, i divide it into small, workable batches and process as described below.

first of all, when diluting your soap [paste], it dissolves much more easily in a lower 25% dilution, but a higher 30% solution produces a better viscosity response, so start by diluting the paste for a 30% dilution.
after the soap [paste] has all dissolved, and the borax solution is added, determine from the resulting thickness, how much cooking is required.
place the soap (in the pot) on low, but direct heat -- no need for a double boiler. bring the soap to a rolling boil, stirring constantly to prevent the soap at the bottom from sticking and burning. when the soap starts to "foam" and expand, turn off the heat and let the foam subside. as the soap cooks, the water evaporates and the mixture becomes more concentrated, and consequently, thicker. do this once if the soap needed only a bit more thickening. repeat [bringing the soap to the foaming stage] if more thickening is needed. remember, it will be thinner when hot, so you won't be able to judge the thickness until the soap has completely cooled. this is something you'll develop a feel for after you've done it a couple of times. when the soap cools, if it's not thick enough, cook it some more.
do be careful to not get it too thick. (you're aiming for the consistency of room-temperature honey.) if you evaporate too much water, the soap will be too concentrated and upon cooling it will start to cloud or even congeal. this is the soap trying to revert back to a paste. if this should happen, not to worry, simply add a small amount of water to thin it. the operative phrase here is "a small amount". if you should over-thin it, simply cook it some more.

lastly, add your fragrance. you'll want to do all your cooking before adding any fragrance as this would "whoosh" all the scent away.
in addition to concentrating the soap, you're also concentrating the amount of borax in the soap, which subsequently lowers the cloud point of the soap solution. therefore, be aware that the addition of your fragrance, (especially essential oils), might cloud the soap, even though the amount is within the 2% range. for the sake of preserving transparency, you may be forced to use less fragrance and/or use fragrance oil. this is another reason for not getting the soap too thick.

the whole point of this exercise is simply to increase the concentration of the soap "after" you've added the borax -- the borax allows the soap to accept the higher concentration. this will result in a soap that thickens the way ms. failor indicated it should.
note: the cooking, the boiling motion, will incorporate lots of air bubbles in the soap and you'll have to let it sit (a couple of hours) for all the air to rise to the top. the resulting head of foam will eventually dissipate.

more about using borax: in addition to allowing the soap to accept a higher concentratrtion, the borax allows the soap to accept "stuff" into solution that would normally cloud the soap.
think about the test for excess fatty acids. the hot solution starts out clear, but if there are fatty acids present, the solution clouds upon cooling. this is because the hot solution can absorb more than a cold solution. as the solution cools, the fatty acids come out of solution ...and cloud the soap. but once in solution, borax allows the soap to hold the fatty acids in solution even when the solution cools. so if you do have a cloudy solution due to excess fatty acids, heating* the solution (after you've added the borax) will allow the excess fatty acids to go (and stay) into solution, in essence clearing up the soap. the soap remains clear upon cooling.
(*this is just to get the soap hot, you don't need to bring it to a boil. you'll be able to see the mixture clearing.)
this comes in handy if, instead of over alkalizing, you should make the soap paste with a slight superfat (2% - 3%). normally this will produce a slightly cloudy dilution, but after adding borax and then heating the solution, the fatty acids go (and stay) into solution and you have a clear soap. of course there are limits to how much the dilution will absorb even with the borax.
note: soap with excess fatty acids won't thicken, even if the borax is successful in eliminating the cloudiness.
this is also useful if you should make goat's milk soap -- (milk added during the soapmaking process). this soap will always have some residue from the milk solids that cloud the soap slightly. one method is to let the soap sequester. the residue will settle to the bottom. you then siphon off the clear soap. however, if you heat the soap dilution, after adding borax, these soap solids go (and stay) into solution and you end up with a clear soap (albeit amber-colored) ...eliminating the weeks of sequestering.