When creating receipts with Blue4est, Koehler does not rely on a chemical reaction through heat, like many others do, but on a purely physical reaction. While printing, the functional layer of the paper becomes transparent under the influence of heat and the black layer underneath becomes visible. This is one of the reasons why the print results are durable for up to 35 years. Another reason is the high resistance to external environmental influences. Among other things, it is heat-resistant up to +50 °C, withstands non-condensing humidity of up to 80% and is light-resistant up to 16,000 lux.