The Steiner Tunnel Test – also known as ASTM E84, tests the flame spread rating of Interior Finishes and Building Materials, specifically wall and ceiling finishes.
Building materials are rated from A – C, with A being most fire resistant. These ratings are used by building codes to specify the minimum rating for materials that can be used in a area by occupancy type.
Take care to not confuse this with ASTM E648 Critical Radiant Heat Flux test for Interior Floor Finishes.
Class A I flame spread 0-25
Class B II flame spread 26-75
Class C I flame spread 76-200
Image/video courtesy cjtyra