I want to follow along with the idea that Jane has brought up. The ware, over time, can absorb impurities from the environment. Buffalo earthenware is very porous, and the crazing in the glaze allows "dirt" to get into the clay body, turning it darker -- definitely off white. I once showed a bone china saucer to Geoffrey Godden that had turned yellowish. He told me that it had gotten "dirty" over the years. Smoke in the air is a potential hazard to keeping china white. Bone china usually has a tighter glaze, so I was surprised that it can even happen with those pieces.
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