Quoting John Palmer in "How to Brew", he has this to say.
"The analogy to a tea bag is a good one in that if the grain is left in for too long (hours), astringent tannin compounds (a.k.a. phenols) can be extracted from the grain husks. The compounds give the wort a dry puckering taste, much like a black tea that has been left to steep too long. The extraction of tannins is especially prevalent if the water is too hot - above 170°F. Previous practices regarding the use of specialty grains had the brewer putting the grain in the pot and bringing it to a boil before removal. That method often resulted in tannin extraction."
While the temperature is important, you still cannot extract tannins without a high pH, or at least not is sufficient quantity to make your beer "puckery". Again from "How to Brew",
"Water chemistry also plays a role in tannin extraction. Steeping the heavily roasted malts in very soft water will produce conditions that are too acidic and harsh flavors will result. Likewise, steeping the lightest crystal malts in hard water could produce conditions that are too alkaline and tannin extraction would be a problem again. In this case, the terms Hard and Soft Water are being used to indicate a high (>200 ppm) or low(<50 ppm) level of carbonates and the degree of alkalinity of the brewing water. "
That implies that both high temperature and high pH are necessary. I have found this information elsewhere too but can't put my finger on it right now.