Impression Materials: Pesky Air-inhibited Layer!

Here’s a tip I learned the hard way—twice! Before I used preparation guides and reduction guides for veneer and crown preparations I would make my provisionals before I took my impression. Then I could measure the thickness of the provisional and be sure I had reduced enough tooth structure so my ceramist had adequate room. It is still a great way to check reduction, however, it also can affect the final impression if you aren’t careful.

Impression Materials and Air-Inhibited Layers

Vinylpolysiloxane impression materials are very sensitive to the air-inhibited layer that is present when bisacrylics set. If provisionals are made directly on the preparations the air inhibited layer remains on the preparations and will likely affect the set of the impression material. The images within this article show an impression that didn’t completely set and a model that was poured from another impression that I thought was okay, but when I saw the stone it was obvious something went wrong. If the provisionals are made directly before the impression is made, pumicing the preps aggressively and then wiping them with an alcohol sponge and rinsing thoroughly can protect the set of the impression.