This is on page 276-277 of Hanna Winsnes’ cook book:
Dried fish for ‘luting’ is difficult to buy as it can neither be too big or too small. The large ones are often so hard that they won’t turn to jelly, and the small ones will over time disintegrate during the ‘luting’ process.
So you need to check that it is of medium size, that it is free of mites, not yellow and not round but spotted. Round cod can rarely be ‘luted’ soft. You often get common ling instead of code; you should try to avoid this as the ling becomes rubbery and slimy during the ‘luting’; it is recognized by the broader tails and the light skin. For lye, preferably use ash from beechwood, but as it is only available in a few places here in Norway, birch tree ash is the second best, followed by all other ash from leaf trees.
If you need to use spruce or pinewood ash, you will need four times as much for the same water.
Four heaped pots of birch ash and half a pot of lime is boiled in a bucket of water holding 12 to 14 pots; this is for two fish that have been skinned and cut, and which only makes one portion for six to eight people. This is poured on when it is lukewarm like freshly squeezed milk. But the dry fish must first have laid ten to 12 days in water, which is changed every other day, then leave it for two days in the lye and 24 hours in water after the lye before boiling. If you want to ‘luter’ four two occasions, then the fish that is to be kept, lay in a mixture of lye and water until the evening before it is to be used for dinner. Lutefisk is eaten either with churned butter with mustard or with melted butter, and the mustard is passed around.