it always kind of amazes me how much trouble people have with coin mokume, its really easy.
here's my method. with a gas forge.
step1:get some quarters and some copper pipe like 1" dia.
step2:lightly tap the pipe until it is roughly rectangular at the opening.
step3:fit as many quarters into the opening of the pipe as you can. i usually get 5.
step4:tap the quarter filled end of the pipe with a hammer to tighten everything up a bit.
step5: cut off the excess pipe with a hacksaw
step6:put the little bit of quarter filled pipe in the forge until it gets to an orange heat
step7: tap it down to further tighten it up some, it doesnt weld, but it makes it easier to handle
step8:heat the billet to a bright yellow/white heat, until you begin to see the billet bead up slightly on the top and and sides.
step9:tap the bilet lightly to set the welds.
step10: repeat 8 and 9 until its nice and solid.
from there you can forge it, cut it, fold and weld it to increase the layers, twist it, accordian cut it, whatever you want. just work it at a bright orange down to a dull red, and every few heats re heat it to welding temp to keep the welds healthy.
no cleaning involved unless the quarters are really grimy. so far i've had one failure, and that was because i didnt heat it up enough.
nickels are pure nickel and so arent very exciting to forge weld, and pennies can be dangerous. if theyre after 1982 then they are copper plated zinc, which can cause fume fever if heated in a forge