These are high temperature stoneware mugs that have been bisque fired, glazed and then bisque fired again to cone 02. This is done so they can be handled without damage (they are being shipped to another location for firing to cone 10R). The glaze is quite durable at this point and would be difficult to damage.
Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it.
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