Both denatured alcohol and propylene glycol are in a class of chemicals called alcohols.
As others have said, denatured alcohol is ethyl alcohol plus something that we can not drink (often acetone or methanol) This is so that it is not taxed as drinking alcohol.
Clear soaps can be made with ethyl alcohol because the sodium salts of the fatty acids in the bar soaps are soluble in the alcohol and stay in solution even after hardening (a bar contains crystalized salts which make it opaque)
Technically, you can melt soaps made with ethyl alcohol; however, due to the low boiling point of the alcohol they will not be repeat-ably meltable and after the first time you melt it, some of the alcohol will evaporate and the soap will not be as clear.
This is where propylene glycol comes in. Propylene glycol (PG) will also produce a clear soap (although sometimes not as clear) and will allow the soap to be repeatedly melted. PG has a boiling point of 370 degrees Fahrenheit. This allows you to melt the soap many times and still retain its meltable characteristics along with its clear properties.
There are pros and cons to each.
Hope this is the right place for this thread! Looking for a good alcohol to use instead of Propylene Glycol in re-meltable transparent soapmaking, mostly because I probably can't say my soaps are all-natural if I use PG. Reading this thread, it sounds like ethanol is fine as long as I don't need to remelt, but since its boiling point is 172.8F/78.2C, it'll evaporate off in a microwave or a crock pot if I'm not careful (while remelting or while cooking it the first time). If PG is better because it has a 370F boiling point, I wonder if Propanediol 1,3 (made from corn) would work because of its' 415.4F/213C boiling point. I spotted this transparent soap at a local natural foods store this week, with "non-petroleum glycol" in the ingredients...
“Propanediol” is an ambiguous name for a compound (no positions given). Wikipedia links to a disambiguation site that links to both 1,2-propanediol (“propylene glycol”) and 1,3-propanediol (“trimethylene glycol”). Either one is rightfully called “propanediol”. From the inscripton on that label, it is not evident which of the two the manufacturer of this soap added.