The outermost layer of a tooth is the enamel. This is a thin layer that partly covers the tooth and forms a wrap for the exposed crown. Following that is the dentin. The dentin is very similar to the enamel except it is not as hard and as shiny and while the enamel is generally white in appearance, the dentin is not. Extrinsic tooth stains forms in the enamel while for the most part, intrinsic stains that show trough the enamel occur in the dentin.
First, the tooth enamel is a highly mineralized very hard part of the tooth composed of strand of rod-like crystalline materials. These rods while very tightly packed have microscopic gaps in between them. The density of the crystalline rods differs from individual to individual and explains the reason why some tooth enamel are tougher than another person's enamel. Nonetheless, all most all of us are born with bright shiny teeth. Even the permanent tooth that follows later invisibly white is cast.
Through everyday wear and tear, the gaps between the rods fill up with debris, tannins and other coloration that stains it. Anything that is put in the mouth will leave its mark trough forming a pellicle film over the teeth which initiated in the pores between the rods. While brushing rids most of the materials that are accumulating, not all of it is removed. Over the years, the accumulation of the debris forms layer upon layer of film that show eventually as stain.
The good thing is that while the staining agents worked its way through the pores in the rods, the whitening agents also use that access to free the teeth of the stains through oxidation reaction. For this two main whitening agents are used, the hydrogen peroxide which is a strong bleaching agent for many bleaching purposes and the much milder carbamide peroxide.
Although hydrogen peroxide is usually the bleaching agent used in in-office procedures it could also be used at home for those who do not have hypersensitivity in the gums or in the teeth. For most practices though Carbamide peroxide is used for home whitening kits. Carbamide peroxide although slower acting also works very much like a hydrogen peroxide. When in the mouth, carbamide peroxide breaks down into urea and hydrogen peroxide with hydrogen peroxide being the bleaching agent.
The dentin on the other hand could also catch stains. Many stains when allowed for a long period of time could get into the dentin forming stains that could show through the enamel. Other factors which could easily get through the enamel to the dentin are cracks in the teeth suffered through a trauma to the tooth, grinding, aging and other accidents. Blood leeching inside the tooth will also tend to stay there and form a stain.
While extrinsic stains are easier to remove, intrinsic stains usually needs more involved professional supervision. The basics though are the same. A bleaching agent will be applied and over a period of time, stains disappear through the stains reaction to oxidation.
