Er... would you like to expand? You aren't giving much away so far.

As an answer to this: Have you ever noticed that an old bar of soap doesn't lather up as well as a new one? Curious1's soap has stopped breaking down dirt, and Curious1 wants to know why: 

Apparent "latherability" is a measure of "how much soap you can get to dissolve in your wet hands" per unit time.

2 factors affect this:- total surface area of the bar and how compacted the soap is at the present surface.

A bar that has halved in length, will have only one quarter the surface area of the original bar.

Bars of soap are made by extruding soap, then cutting it to length and then stamping in a compression mould (although the last 2 steps are often done simultaneously.

In the extrusion process (especially at the point of exiting the die) the outer "layers" end up being less compacted than the center. The stamping may also add to this, especially if there is some extrusion of excess soap.

Hence APPARENTLY less effective soap.

In the case of an aged unused bar, the water in the surface of the bar evaporates over time and causes less speedy dissolving of the soap

There could be several reasons. As a previous correspondent has noted, the manufactring process itself could be to blame, creating more compacted areas in the bar of soap. It could also be that the original questioner lives in a very hard water area, and that the calcium fatty acids salts (which are terrific defoamers in their own right) form very quickly. I think that the most likely explanation is to look at what the soap is made of. You can make bar soap from a variety of fatty sources. The most common are Palm and Tallow. In all cases a chelate such as EDTA will be addeed to the formulation to help counter hard water. If you have a palm-based soap, then there's likely to be a good proportion of lower carbon-chain fatty acids present, most notably Lauric Acid. This aids in generating foam/suds much more so than the higher palmitic or stearic acids. It is also more water soluble than the higher fatty acid sodium salts, and will therefore dissolve from the bar more easily. This ,as well as moisture loss, is why some old bars of soap get cracks in them and begin to lose their foaming power.

Peter Finan

Haworth

West Yorkshire