File:Alcohol Test 3.jpg After it was clear that something strange was happening when light guides were placed in ethanol concentrations between 60-80%, we tested 10 more light guides. There were two fibers in each concentration of 60%, 70%, 80%, 90% and 100%. This test shows swelling around the 80% concentration mark. The concentration which causes damage to the fibers is higher when the solutions are not heated.