Good news? I don't think it's the Graniteware.
Bad news? Two attempts, two failures. ARGH.
It's getting overacidified. Milk is normal (6.6 pH). I add what I thought was the correct amount, 1.5 tsp diluted in 1c distilled water (per 1 gallon of milk). The first one, in my Calphalon that I had used countless times, is overacidified and curdled, quickly. pH dropped to 5.5.
So, I thought, perhaps I added it too quickly. The next try, I did it in the Graniteware -- added very very slowly. 2/3s of the way through adding the acid,the pH already had plummeted to 5.8.
Obviously, I could add less, but what is going on???