For print mounting, we use a 3mm thick closed-cell PVC foam board, which is often referred to the brand name Sintra. The PVC board is very rigid, durable and solid, yet it is very light. Sintra is acid free and has minimal out gassing for Archival use. We know a lot of photographers mount to a substrate referred to as Styrene and we see our board as an upgrade to that.
To adhere your fine art print to the PVC board, we use a double sided archival mounting adhesive that is acid free.
We've chosen both of these materials carefully after a lot of research because we feel their quality matches the high level printing we do. Since our prints are cotton based, we want to pair them with a mounting substrate that feels substantial, yet will stand the test of time.