The present invention is a number of methods and devices for tooth rejuvenation and hard tissue modification comprising applying a layer of a peroxide-free composition to a tooth surface. The applied composition comprises an aqueous solution of one or more edible acids. The composition has a pH selected from the range of about 0.5 to 5. After the treatment the composition is removed from the tooth surface. In various embodiments of the invention the pH of the composition can range between about 0.5 and 3, a narrower range being between 1 and 1.75. Hard tissue rejuvenation comprises forming a post-treatment layer having a composition which is different from that of the hard tissue by selectively heating a porous layer on the hard tissue.