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Besides, can warm and white batting be used in microwave?
No, there is not a right or wrong side but there is a "scrim" side. When manufacturing Warm & Natural or Warm & White, the cotton fibers are layered onto a scrim - a thin nonwoven substrate material. The Warm Company does not recommend cotton batting, polyester batting or blended batting for microwave use.
Similarly one may ask, what fabric is safe for microwave?
Natural fibers - cotton, linen, hemp, silk, etc. - should be okay, as long as they don't get heated for too long. I'd avoid synthetic fabrics, like polyester, since they're plastic and such.
Polyesters can outgas some noxious toxic fumes when heated under an iron. No telling what happens in a microwave. . Best to stick with all cotton.