Sealing by heat; usually boiling water or very saturated steam, forms a hydrous aluminum oxide film. Water is necessary for this reaction to proceed (dry heat won't work). The temperature of this reaction is below but close to the boiling point of water. This form of aluminum oxide is less dense than the anodize, hense it takes up more space, and fills the pores. The hydrous Al oxide forms on the outer surface and in the pores, it doesn't normally displace any dye in the pores. As it forms, it plugs up the open end of the pores. Being aluminum oxide, it is as chemically inert as the anodize, but a little softer.
IMHO; this method of sealing is superior to chemical sealing, which plugs up the pores with a chemical precipitate, which is not as inert. Heat sealing (more accurately wet heat sealing) however, takes longer to do, and has a higher fuel cost.
|