I am thinking about making a dessert that calls for baking in ramekins @ 450 degrees. Can I use my Johnson Bros. ironstone coffee cups at that temperature?
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I'm not familiar with that particular dinnerware. Check the manufacturers website for whether it is considered a stoneware product.
In terms of pottery, 'stoneware' is high fire clay and glaze and should be fine at that high a temperature (although not, of course) on a stove top. Earthenware is a lower fired product and I would not recommend that temp.
It should be fine. I have used my coffee mugs in the oven plenty of times, from french onion soup to reheating chili. And mine are cheapies from Walmart.
I think you can. Does it say on the bottom of the cups?
If the cups are oven safe, absolutely.