Here's a Pyrex CD 234 I recently aquired. It has a swirling pattern that goes around this entire insulator of the skirt. I know this insulator came down off a cross within the last couple of weeks, so what it is surfaced with is not from the acidity of any soil. I'm in agreement that carbon collected on the molds to create the swirls shown on this one. However, being acidity of soil was mentioned, another questioned occured to me. That question would be, what would the effects of Acid Rain be on insulators? I'm quite sure these were up in the air when the discovery of acid was made and how it was not good for the enviroment.