A viewport assembly may allow viewing of a retort chamber while protecting the viewer from heat of the retort chamber through a high-temperature-resistant window in an end wall of the viewport assembly. An access port in the end wall may allow use of tools, such as a welding device. An insulated door between the viewport assembly and the retort chamber may enhance protection and heat retention. The assembly may enhance retention of inert atmosphere in the retort chamber as well as heat, facilitating work on superalloy articles.