The recordable layer typically contains a polycrystalline alloy made up of a mix of silver, indium, antimony and tellurium. The recordable layer of a brand new CD-RW is entirely in the transparent polycrystalline form.

To create a non-reflective area that a read laser would identify as a binary 0 the phase-change material is heated to above the melting point, around 500-700 °C.
In the liquid state the atoms are free to move around, losing their polycrystalline structure and becoming amorphous.
If the liquid is then cooled rapidly below the crystallization temperature this structure will be frozen in, leaving an opaque mark. To achieve this rapid cooling a dielectric layer is placed either side of the recordable layer which removes heat effectively.