"...So if we end up with a lye heavy batch, will it eventually be safe to use? Are we now putting too much emphasis on zap results after only a couple of days cure?..."
In my opinion the answer is "yes" to both questions, it will be, but this is just my opinion, not established fact. So take this with a big grain of salt (uh, soap), please!
(A stage whisper to The Gent ... I'm basing this opinion on results from soap made with a "normal" recipe as well as Anna Marie's lye-heavy castile recipe with lots of extra water.)
I spoke with Kevin Dunn who wrote the Scientific Soapmaking book about this. He confirmed in our email conversation that regular soap bars he has tested with up to 5% excess lye have done the same thing -- the excess alkalinity (too much lye) in these bars gradually drops to zero with time. There's a brief write-up about Dunn's experiment in his Scientific Soapmaking book in his chapter on alkalinity testing -- see Example 15-1, pg 250.
A 5% lye excess would be pretty seriously irritating to the skin to the point one would not want to use it in the bath, so this is an intriguing finding. Most people would rebatch a soap like this, but simple time and patience might be an alternate treatment.
Dunn only tested twice -- right after making the soap and after 11 weeks of cure, so that doesn't tell us precisely how long is long enough. The only thing that we know is it took at least 11 weeks for his "normal" soaps to drop to an acceptable alkalinity. That said, I'd say it took roughly that much time for my two batches of super-lye soap to become zap free and mild to the skin during hand washing tests at the sink. But I didn't do alkalinity testing -- just zap and hand washing.
Dunn and I speculated on what was causing the drop in alkalinity.
One thing that The Gent mentioned is that water, as it evaporates from the bar, will transport excess lye to the surface of the soap. When the lye reaches the surface, it will react with carbon dioxide in the air to make soda ash. Many of the super-lye soaps do show a LOT of ash formation, so I'm pretty sure this is one way the lye becomes neutralized.
But my trusty calculator tells me that soda ash formation alone might not be the whole answer. The ash should be really, really thick on my super-lye soaps if ash formation is the only way the excess lye is being neutralized. Dunn and I talked about ways to test what is going on; I need to set these tests up and get them going. (Summer activities have interfered with investigations into soap chemistry!)
I want to add a caution -- I'm talking about otherwise well-made bars here -- not ones that have separated, are weeping weird fluids, or otherwise do not appear to be properly made. I'd rebatch if it looks like the soap has more issues than just being somewhat lye heavy.
