В этом документе пишут про разницу в химии следующее. Для однократно записываемых болванок на отражающий металлический слой используют золото, серебро или сплав серебра; для перезаписываемых — алюминий. Свойствами используемых материалов отчасти и обусловлена разница в свойствах носителя.

(Для пробуждения интереса у не читающих по ссылкам анонимов я скопирую этот текст сюда)

3.3.2 Metal Layer in R Discs

In R discs (CD-R, DVD-R, DVD+R), gold, silver, or a silver alloy is used for the reflective layer. Silver is slightly more reflective than gold but can lose reflectivity with corrosion on exposure to adverse environmental conditions. Silver corrodes through reaction with sulfur dioxide, an environmental pollutant that can migrate through the disc with moisture. Gold is noncorrosive, very stable, and longer lasting, but it is also expensive. Either metal should outlast the dye. Aluminum is not used with these discs because it can react with the dye in the recording (data) layer.

3.3.3 Metal Layers in Double-Layer DVD-ROM Discs

DVD-ROMs can be manufactured with two reflective metal layers that allow the laser to read data from both layers using one side of the disc. These “double layered” DVDs provide up to four times the capacity for content (video, audio, computer applications) as do “single-layered” DVDs. The laser beam must pass through a semi-re- flective metal layer to read data from a fully reflective layer. The outer metal layer (silicon, gold, or silver alloy) is semi-reflective; that is, it reflects back some of the laser beam and allows some of it to pass through to a fully reflective layer (aluminum) and then reflect back. Both parts are thus reflected to, and detected by, the photosensor in the laser head, which focuses on one layer at a time.