Potter's soap is an interesting material. It only does one thing; it is used to seal plaster. But, it does it like no other material can.
If you are doing any type of casting, or are making molds, you will most likely end up using plaster at one time or another. Plaster may be used to make a sculpture, or a mold, or both. You may also use plaster to make a mother-mold to back up another material. In any case, plaster is one of the most common materials used in an artist's studio.
One characteristic of cured plaster is that it is porous. This can be useful at time. Cured plaster can be briefly soaked in water, absorbing enough water so that hot wax can be poured onto the plaster without the wax sticking to the plaster. On the other hand, some materials, such as more plaster, Urethanes, or Silicones, require a dry, sealed surface. In this case, we need to seal the plaster. This is what Potter's Soap is used for.
Potter's Soap really is a soap. When you buy Potter's Soap, it comes in a concentrated, goo-like form. Take one part (by volume) of the goo, and mix it with 4-5 parts water. Some people like to let the mixture soak for a couple of days, other dive right in with a Jiffy Mixer and an electric drill. Mix until creamy smooth. Once mixed, the material can be stored in a sealed container indefinitely (although additional mixing may be needed later).
To use Potter's Soap, your plaster needs to be cured, but not dry. If your plaster has dried, soak it for 5-10 minutes in water. Pour a little of your Potter's Soap mixture into a small container, and with a soft brush, whip and beat the mixture until it becomes frothy. Then, using the same brush, apply the frothy Potter's Soap foam to your plaster liberally. Allow to dry. Apply a second coat the same way, allowing it to dry, then a third coat. Finally, polish the surface with the soft, dry cloth. You should see a glossy surface on the plaster where you have applied the Potter's Soap.
Potter's Soap (potassium oleate) reacts with the plaster (calcium sulphate) to form calcium oleate, or a 'soap scum'. The 'scum' that forms on the surface creates a nonporous waxy film, sealing the plaster.
Once sealed, new plaster may be poured or cast directly onto the soaped plaster, with no additional sealants or release agents needed. Other materials, such as Urethanes and Silicones, will require some form of release agent.
"So, if Potter's Soap is only for sealing plaster, why can't I use other sealers, such as shellac, wax, or Vaseline?"
One of the strongest features of Potter's Soap is that it reacts with plaster on a chemical level. This results in a sealed surface with no buildup. There is no loss of surface texture or detail with Potter's Soap. Other sealants, such as wax or Vaseline, can have a significant thickness, resulting in the loss of surface texture or detail.
Another feature is that once the plaster is sealed with Potter's Soap, there is nothing to transfer to the next material. This means that new plaster poured or cast against a soaped surface will not pick up or absorb any trace of the soap. This retains the new plaster's ability to absorb water.
So, there you have it. Sure, Potter's Soap is a 'one trick pony'. But, no other material can do this trick. Try some, and see for yourself.