If you use the "96%" moonshine, one usually tends to be more generous on the coffee than on booze. With normal 60% "polsprit" (legally bought spirits from Vinmonopolet) it is custom to mix up to half and half. A traditional "recipe" is to put a coin in the bottom of the cup, pour the coffee until it is no longer visible, and mixing with alcohol until it reappears. The recipe is often claimed to be a hoax, as the coin will not reappear in a cylindrical coffee cup. This phenomenon is explained by the Beer–Lambert law stating that the absorption of light is proportional to the concentration. As such in order for the recipe to work one will need a cup with a significantly wider top than bottom which will allow for the concentration to decrease faster than the column of liquid increases. Coffee used in karsk is often weaker than regular coffee, and at rural celebrations in the Trøndelag region, it has been customary to serve half cups with "thin coffee", since it is expected that most people carry a flask of moonshine with them, to add to the cup. If the Karsk becomes too strong, it is possible to decrease the alcohol content, inflame the surface of the alcohol-fume with a lighter or match. The hotter the coffee, the easier the alcohol will burn. Cold karsk will not ignite until the concentration exceeds 60-70%. The Karsk will also taste subjectively stronger the hotter it is. It is recommended to taste towards a desired concentration by increasing the amount of ethanol gradually, since the burning alcohol will be lost.