Here is the thread with Irish Lass' posts:
http://www.soapmakingforum.com/showthread.php?t=46114
I'm a LS newbie, so I'm still learning, but here's what I can contribute:
My understanding is that mixtures of soap and water are quite different than mixtures of many other materials, such as sugar and water. Sugar dissolves in water to make a "true" solution, in the chemist's specific use of the word "solution". A solution to a chemist is a mixture of individual molecules of one thing (sugar) floating around in another thing (water).
Soap CAN make a solution of soap in water -- meaning individual molecules of soap floating around in water. A true solution happens when you mix a lot of water and a little bit of soap, as when you squeeze a squirt or two of LS into a sink full of water to wash dishes.
But soap can make mixtures with water that are not true solutions. These mixtures are called colloids. A colloid is any mixture of soap and water in which
droplets of one material are floating around in the other. A droplet is much larger than a single molecule, so a colloid looks and acts differently than a true solution. What is most important to LS makers is that a colloid will be thicker than a true solution. Depending on what is in the colloid, some will look cloudy (hand lotion) while others are translucent (egg white).
When liquid soapmakers try to dilute LS paste into a pourable product, they are trying to find a very specific "sweet spot" where the colloid viscosity is just right. The problem is this sweet spot varies depending on the recipe, temperature, time, and other factors, which is why LS makers try to sneak up on the proper dilution for each batch. The sweet spot is right where the colloid structure is switching over from paste (water-in-soap colloid) to a liquidy version (soap-in-water colloid).
Soap-in-water colloid: A mixture of soap droplets floating around in water. Usually happens when there is more water than soap, like in a diluted LS.
Water-in-soap colloid: A mixture of water droplets floating around in soap. Usually happens when there is more soap than water, like in a concentrated LS paste.
The gray area between these two types of colloids is when the soap and water mixture contains
very roughly the same amounts of soap and water. This area is where you are likely to find the sweet spot, but there are issues that make it a challenge to find.
One reason why the sweet spot is hard to find is that the soap doesn't quite know whether it wants to be a soap-in-water colloid or a water-in-soap colloid. It can tip from one to the other very easily. If you don't dilute quite enough, the paste simply won't make a consistent, smooth, pourable product -- you'll have chunks or a skin that will never dissolve. If you dilute a little too much, you can easily get a runny, too-thin product. (And if you dilute even more, the LS will transition into a true solution and be very runny.)
The transition point is also not an exact point in the dilution scale. If you're diluting a LS paste, you may have to dilute a tiny bit more than you might like to get the colloid to shift from water-in-soap (too thick) to soap-in-water (thinner, pourable). If you try to thicken a diluted soap by removing water by evaporation, you will most likely find you will have to evaporate a tiny bit more water out of the mixture than is ideal to get the colloid to thicken up enough. And if you dilute too fast, you can overshoot the sweet spot because the structure of the colloid can change with time, ultimately stabilizing in a form that is thinner or thicker than it was originally. That's why some LS thickens up (or thins out) just sitting on the counter.
All this "sloppiness" is called hysteresis, for those who want to learn a fancy $5 word today. And this is the reason why separate thickeners have a place in LS making, especially for recipes that have a small sweet spot.
