For soap makers Iodine is an aspect of measurement that appears in many soap making calculators that help gauge the stability in a hand made bath of soap. Specifically, iodine is the measurement of the polyunsaturated quality of the fatty acid profile. The higher the number the more polyunsaturated. This a a rough measure of how easy it is to saponify the fats in a soap recipe. It is not, by any means the most valuable measurement, and was widely used at the beginning of the century for large commercial soap making companies as a tool of measurement for large batches. This was meant to be a one number, quick glance of the quality and shelf life of the product.
Since higher saturated fats in your soap are usually made with harder oils logic follows that the lower the Iodine number in a soap recipe the harder the bar of soap will be and the less the conditioning qualities and vice versa.
For hand craft and small batch soap makers, it may be helpful to know that the lower the Iodine number in the recipe the faster the soap will trace during the emulsification process.
For designs that require a thick pudding texture an Iodine number near or below 50 is fine, but if the goal in the recipe is to get light wispy swirls the recipe may work better with a higher Iodine number, above 50, but under 65 to keep the recipe balanced and the integrity of the bar to keep a stable shelf life.
Definition: number of grams of iodine that will react with the double bonds in 100 grams of fats or oils.