Allow them to heat gradually. Do not take from a cool place and put in the direct sunlight, they may crack. The moisture will get under the glass. It is highly corrosive of brass if stored together with brass and other metals. Plastic buttons should be stored alone.
Some plastics contain carbolic acid.
With time those buttons will be destroyed themselves as well as any metal buttons that come in contact with them. They will also destroy the metal shanks of other material buttons. Metalized plastic should also be stored alone. It too is highly corrosive of brass if stored with brass. Is also corrosive if stored with other materials.