Zany, you may not have had your Pyrex shatter on you while soaping, and I hope you never do, but other soapers have, and it was a pretty scary and messy experience for them- one they do not care to ever repeat. I for one do not consider cautioning those against using it as a container for mixing lye as scaremongering, but rather being a responsible soaping mentor to those new to the craft, especially when there are much less potentially dangerous lye mixing alternatives available.
Here are some examples of folks here on the forum who have experienced adverse reactions with their Pyrex. I'll begin with a former modmin of mine and a very experienced soaper who you may or may not remember (carebear). She had her Pyrex mixing container explode in her sink just a few minutes after mixing her lye solution in it. Thankfully, it was in the sink (not on her counter) and she wasn't standing right over it when it happened.
From forum member Raha from 2010:
Raha said:
Dear Soap Makers,
Today I write for your support. I always try to be so careful when dealing with lye--as I have read a few horror stories. Well, I made a big mistake today...while making my green tea soap, I poured hot green tea in with lye...I feel so horrible for making such a big mistake...the lye bubbled up like a volcano and I stepped back and then the whole glass jug exploded. I am so so so grateful that I didn't get hurt (except for a minor burn on my thumb). I just finished cleaning the minimum and plan in a half hour or so to continue the mass clean up.
Anyways, thank you for your support and just let me share my frustrations...
Gratefully yours,
Raha
From forum member Deda from 2010
Deda said:
Earlier this week I had a Pampered Chef glass batter bowl [which is made from tempered glass] explode just sitting, clean, dry and unused on the shelf. I didn't use it for soap, only pancake batter. Can you imagine the havoc wreaked if it had been filled with lye/water.
Forum member Paillo from 2011:
Paillo said:
i would be scared to death to use a glass or pyrex container for my main mixing bowl. i use glass for my smaller containers -- swirl colors, milk mixtures, etc. but last week we had a large pyrex container shatter in the sink. mercifully it wasn't soap, but there were scary sharp shards of glass everywhere, and one of us or one of the pets underfoot could have been seriously hurt.
Forum member Candybee from 2013:
Candybee said:
From my own personal experience I had a large pyrex measuring cup shatter on me one day. I wasn't using it for CP at the time. I had been making M&P soap in that glass for about 5 yrs before it finally broke. I was so surprized and had not imagined that such a sturdy popular brand of glassware would shatter on me but it sure did. So when seasoned soapers tell me that lye can etch the glass that tells me it weakens the glass and yes it will eventually shatter. Could take a few weeks or a few years but I would not want to be there if it shattered with a lye mixture in it.
Forum member BlueberryHill from 2015:
BlueberryHill said:
I just had my Pyrex glass measuring cup explode as I was pouring lye into it! Thank goodness it was in my laundry sink, where I always mix my lye water, and I was all safety geared up! I'm shocked because I used it last week for the first time and it was fine. I thought I was being safer using the Pyrex than the plastic container from the dollar store. I hadn't read the article in this thread before. Beware everyone.
Forum member Newbie from 2016:
Newbie said:
I am one of the people who has had pyrex shatter. Thankfully, it shattered while in the dishwasher instead of all over my kitchen. Lesson learned.
There are also a good handful of soapers over at the Dishforum that have had it happen to them as well, but I'm not about to copy/paste their testimonies here. You'll have to go searching for them over there.
Glass is pretty strong stuff, but there are four things that are its 'kryptonite (if I can borrow from Superman jargon), i.e., things that aggressively attack it and weaken its resistance, causing it to actually corrode and even dissolve layer by layer:
1) Hydroflouric acid
2) Concentrated phosphoric acic (when hot or when it contains flourides)
3) Hot concentrated alkali solutions of at least 9 pH (lye has a pH of 14)
4) Superheated water
Of the above 4 things, the thing that concerns us as soapmakers of course is #3, alkali. Alkalis affect the surface of glass by directly attacking the silica that the glass is made of, leading to its dissolution, little by little, over time as the surface of the glass is repeatedly exposed to the alkaline solution. And the rate of corrosion increases if the hot alkaline substance is agitated, such as when being stirred.
The above corrosion happens to
all glass with the repeated exposure to hot lye solution-
even borosilicate. See the following safety sheet on borosilicate glass....scroll down to 3.3 under Chemical Composition:
https://www.scilabware.com/en/glass-types-properties
"Only hydrofluoric acid, hot concentrated phosphoric acid and strong
alkaline solutions cause appreciable corrosion of the glass." [emphasis mine]
Also this in regards to borosilicate glass
: http://www.camglassblowing.co.uk/glass-properties/
"Resistance to alkaline solutions is moderate and strong alkaline solutions cause rapid corrosion of the glass, as does Hydrofluoric acid and hot concentrated Phosphoric acid." [emphasis mine]
See also the following in regards to how glass corrodes
: http://corrosion-doctors.org/Household/Glass.htm "Glass is resistant to most acids but is highly susceptible to attack by alkaline materials, especially a concentration of OH- ions giving a pH greater than 9.0. The result is an attack of the network forming silica-oxygen (Si-O) bonds, leading to dissolution of the glass surface." [emphasis mine]
While it's true that borosilicate glass is stronger and more resistant and takes a longer time to weaken than soda-lime glass, its still chemically vulnerable to concentrated alkaline solutions and will dissolve bit by bit with every exposure until it reaches the point of no return, aka, 'kaboom!'. The problem is that it's impossible for any of us to know when the shattering day of reckoning will arrive until it actually arrives. You may be willing to take that risk, and that is your choice, but I for one will continue to 'proselytize' about it so that those who are new to the craft and haven't yet been made aware of this issue will at least be able to make an
informed choice.
IrishLass