A test rig that reproduces high temperature and high pressure conditions found in a gas turbine engine for testing materials under these conditions. The test rig includes a combustor surrounded by an upper plenum chamber and an exhaust plenum that receives a hot gas stream from the combustor. A transparent channeling vessel guides the hot gas stream from the combustor over a test specimen on which a material to be tested is mounted. The hot gas stream exits the clear channeling vessel and into the exhaust plenum where the stream is cooled by diluting the stream with cooling air. A viewing portal is located in the exhaust plenum so that the material can be observed through the clear vessel.