What substance is known to create a rich layer under a sealant but should not be applied before etching?

Fluoride is known to create a rich layer under a sealant, enhancing the protective effects against caries while not being recommended for application prior to etching. Etching is critical for preparing the tooth surface to ensure proper adhesion of the sealant. It involves the application of an acid to create a roughened surface, allowing for better bonding of the sealant once it is applied. If fluoride is present on the tooth surface before etching, it can interfere with the etching process and prevent optimal bonding. Therefore, fluoride should be applied after the etching and sealant placement to ensure that the sealant adheres properly and that its sealing properties are maximized.

In contrast, substances like chlorine, calcium, and silica do not perform the same role as fluoride in the context of sealant application and etching processes. Chlorine does not apply to dental practice, calcium, while important for tooth structure and health, is not typically concerned with bonding procedures, and silica is primarily used in dental materials for different purposes, including providing strength and durability.