Biggest cause for a blowout is a micro fracture.2 jars (out of 26) of green tomato salsa blew out on me yesterday in the canners. I was using a pressure canner and hot water bath so I could process 13 quarts at once rather than just 6 or 7. 1 blow out happened in the pressure canner and 1 happened in the hot water bath. The jars were temped and hot packed and went into warm (not boiling but not cold to avoid shock to the glass) water in the canners and were brought to boil/pressure gently - I start at medium heat on the burners and increase it to high over 10-15 minutes. High is necessary to boil such a large pot of water as well as create enough steam to pressurize the pressure canner properly. Heat is lowered a bit (gently again) to maintain boil or pressure. Checking the lids on the blown out jars and the other (not blown out) jars there's no excessive bulging or denting (outward) that would indicate too much pressure in the jars. There's also no sign of damage to the lids on the blown out jars. The cracks extended through (or possibly started from) the bottom edge/corner of both blown out jars. There's a natural weak spot in the curve at the base of the jars but that's also a place where a chip or hairline crack could easily occur from impact. These jars made the trip from TN to Chicagoland as I purchased them while visiting my in-laws last year or two years ago and this is their first use. I'm thinking that rattling during the drive may be the issue and led to hairline cracks in these two jars. Of course I'm paranoid now and checking all the jars for cracks or chips. It could have been a manufacturing fault also. I'm not thinking it was a canning process issue though.
Anybody have any thoughts?
Next largest is a trapped air pocket.