During the polymerization of light-cured composite resins, the hardening of the monomers on the uppermost surface of the material (approximately 10-50 microns deep) that comes into contact with air is inhibited by oxygen in the air; this unhardened, adhesive layer is called the oxygen inhibition layer. This layer is extremely useful when adding successive composite layers in bonding applications because it allows the next layer to chemically bond to the previous one. However, if this layer is left on the surface after the final layer of the restoration is completed, the polishing process can never be fully completed, the filling remains matte, and over time, significant abrasion and brown discoloration will occur on the surface; therefore, before the final light value, a glycerin gel (barrier) should be applied to the restoration to prevent contact with air, and this layer should be completely removed before final polymerization.
