Net weight, and the bar that keeps shrinking

The commonest labeling error in the craft, and it comes from a property of the product rather than carelessness.

A moving target

A net quantity of contents is required on a consumer commodity, and it has to be accurate.

Soap loses water for weeks after it is cut, and then continues to exchange a little with the air indefinitely.

So a bar that weighed 130 grams at the cut weighs meaningfully less at six weeks, and a little less again in a dry shop in winter.

Declare the cured weight

Not the mold weight, not the cut weight, and not the batch weight divided by bars.

Weigh bars from a finished, cured batch and declare from that.

A maker who declares the cut weight is under-declaring nothing and over-declaring everything, which is the direction that matters.

How much margin

Declare a weight the bar will still meet after it has sat in a dry room for months, because that is a real customer's shelf.

Which means declaring a little below the average rather than at it, and treating light bars as seconds.

The cost of that margin is a few grams. The cost of the other direction is a short-weight product on a shelf.

Getting it consistent

A scale with resolution fine enough to see the variation: a scale reading to the nearest 5 grams cannot manage a 120 gram bar.

A consistent pour depth and a cutter with a stop.

And a check weight logged per batch, which takes a minute and is also the record you want if anybody ever asks.

Both US customary and metric units, in the placement the packaging rules require.

References