Since swim could not come to pure Sodium Carbonate to use in washing the elves' spice in order to remove any lye left, he's got some Sodium Carbonate Decahydrate, and was wondering if this could be used instead. Anyone any experience with Sodium Carbonate Decahydrate?

It works fine. If you want you can dehydrate it with heat. The decahydrate is simply sodium carbonate with water trapped in the crystal structure.
A little tip from me if you can't find Sodium Carbonate (Na2CO3, Washing soda):
1. Buy Sodium Bicarbonate (NaHCO3, Baking soda).
2. Put it in a wide heatresistant container, and place it in oven at 250°C/500°F.
3. Stir occasionally and observe carbon dioxide escape as the bicarbonate decompose into the hydrogenfree carbonate of sodium instead.
(2 NaHCO3 + Heat → Na2CO3 + CO2 + H2O)
4. When the carbon dioxide stops flowing, wait a little further.
5. Take it out, let it cool and you're done. You now have sodium carbonate!

West-en, thanks a lot, this seems quite understandable, but then probably swim has to expect as well some water appearing (when the bicarbonate breaks into carbon dioxide and water), or maybe due to the heat it will evaporate together with the CO2. It is nice that you have put down a formula, it makes it even more understandable. Do you know what is the chemical process that happens behind the "washing" of elves spice + nafta/rubber cement remover + NaOH (the lye)+ Na2CO3 (the soda), swim was wondering how exactly the NaOH is washed away?