I think I understand that the salinity is about 35% and that salt helps to harden the bar. Should I leave out the sodium lactate when making this type of soap? My worry is that too much salt could make the bar crumbly.
Table salt (NaCl, sodium chloride) is saturated in water at about 25% concentration -- 25 grams of NaCl plus enough water to make a total of 100 grams of brine solution (saltwater mixture).
A 25% solution concentration is made by dissolving 1 part NaCl (called the solute) to 3 parts water (the solvent). A ratio of 1:3 salt-to-water translates to a proportion of 33% solute-to-solvent. That is roughly the 35% number you've mentioned.
While chemists are comfortable with both methods of stating concentration, people on this forum mostly use solution concentration (see first paragraph). It can be confusing when a person talks about making a mixture in terms of a solute-to-solvent concentration (second paragraph).
I see this misunderstanding come up mostly when people talk about soap made with a table salt brine, so I thought I'd explain the difference in this thread before readers get too confused.
People have gotten odd results when using more than one type of salt in a batch of soap -- and if I recall correctly, the main problem was the soap was overly soft, not overly hard. For that reason, you might be better off by using one or the other of these two salts (table salt and sodium lactate), but not both.
I use sea water in some of my soap. It's only about 3.5% salt. I use it as I would any other water/liquid and my lye concentration is usually 30%. Be aware that the lye solution will be cloudy. I don't use sodium lactate in my seawater soap.