Cavities were observed in the fire resistance limit test of indoor intumescent fireproof coatings

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For some reason, the first post was sent incorrectly in the previous message, and it can no longer be found now. The second post was deleted by the moderator for being repetitive. Change the title and post it again; please be lenient, moderator. Recently, when testing the fire resistance limit of intumescent fireproof coatings, cavities have appeared beneath the char layer on the surface; there are bun-shaped protrusions in the char layer below these cavities. The reason for this is unknown, as similar samples prepared with the same formula did not exhibit such phenomena during previous tests. The samples used for testing were placed directly in an oven at 90 degrees after being coated, first allowed to reach surface dryness at room temperature before being transferred to the 90-degree oven; they were again allowed to reach surface dryness at room temperature and then transferred to an oven at 50 degrees before finally being raised to 90 degrees for baking. I’ve tried all three, and the same phenomenon occurs in each case.