... Having soft water will also reduce or eliminate scum.
As
@Obsidian mentioned, having soft water reduces or eliminates scum.
Without the minerals in the water (that make the water "hard"), the insoluble mineral soaps cannot be formed.
I wonder if its the saturated fat that causes it? Does castille leave white soap scum stains?...
The calcium and magnesium (mostly, but other minerals too) in hard water can form soaps that are white and don't dissolve - soap scum.
Calcium stearate is commonly known as part of the soap scum (family?
), but calcium oleate is also insoluble, according to this
old paper
(see the line "water" on the second page - all are marked insoluable):
http://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC1259507&blobtype=pdf
... But ... is soap scum really the culprit of cloudy glass in the shower? Or is it lime/hard water deposits? We don't live in a hard water area. It seems that anything will make the shower glass go cloudy. My friend had a brand new shower installed, only uses liquid soap, and cleans it after every shower with a window cleaner thingy (like a car windscreen cleaner) and she still has problems.
We don't have hard water either and we have the same issues. It was like that even when we weren't using handmade soaps.
I did a bit of research on this a while ago, and ended up talking to a glass manufacturer. The clouding in shower screens can occur with any cleaner that reacts with glass (similar to the reason why we don't make lye in glass containers) - the glass reacts just a little each time, and for shower screens, this is seen as tiny holes (or "pits") in the surface of the glass. Visually, this looks almost identical to soap scum.
Some cleaners will aggressively pit the glass. A check (to see if it's pitting, rather than scum, is to wipe the screen (when it's clean and dry) with a very light coating of sweet almond oil. If it goes clear, then it's pitting, not scum.
(The sweet almond oil can be left on the screen to reduce the visual impact of pitting, and using less reactive cleaners helps keep the glass in good condition).