Seizing

A fluid batch becoming an unpourable mass in seconds, usually right after the fragrance went in.

The batch turns to a solid

From pourable to something you could carve, with no intermediate stage.

It is not a fault in the soap: the chemistry is fine and the soap will be perfectly good. It is a fault in the plan.

Why it happens

The fragrance. The overwhelming cause. Certain fragrance oils accelerate violently, and florals and spices are the usual offenders.

Temperature: a batch made hot has much less working time.

A water discount, which concentrates everything and speeds trace.

Over-blending, which takes a batch through the stages faster than you can react.

Some additives, clays especially.

What to do in the moment

Stop blending immediately and switch to a spatula.

Abandon the design. There is no swirl now and trying to make one loses the batch.

Get it into the mold: spoon it, press it, level the top with a spatula, and accept a rustic bar.

If it is truly solid, press it into the mold in handfuls and expect air pockets, or divert to hot process, which is an entirely legitimate rescue: cook it through and mold it.

A seized batch is almost always saveable as soap. It is only lost as a design.

Preventing it

Test every new fragrance in a small batch before a production run, and record what it did.

Buy from suppliers who publish behavior notes.

Work cooler, at full water, for anything with a design.

Stir fragrance in by hand rather than blending it.

And have the mold ready, uncovered, before the fragrance goes in. The seconds spent finding it are the seconds you needed.

References