I made a little research and almost all used alka-seltzer and could be only used one time, beiing necessary to put more alka-seltzer to make it run again.
all you need is:
a small alluminium can,
a piece of net,
a pice of MDF,
a glass bottle (colorless is the best)
a felter pen (to paint alcohol)
mineral oil (is the best becouse you can paint it, but you can use olive oil, cooking oil, or similar, but you won't be able to paint it)
a crayon (to paint oil)
a lamp (halogen is best, but you can use any other, but it has to heat a lot)
a ceramic lamp socket (IS VERY IMPORT THAT THE LAMP SOCKET IS CERAMIC, OTHERWISE IT WILL MELT!!!!!!!!!!!!!!)
Step 1: Preparing the bottle
preparing the alcohol
to paint the alcohol, disassemble the pen (mine was green to put the alcohol green), remove the charge and squeze the ink onto the alcohol. if necessary shake it.
preparing the lava
to paint the lava (oil) use a crayon and dissolve it onto the oil. this is a very difficult thing, becouse you have to wet your fingers in the oil, rub them in the crayon and clean the oil in your fingers in the remaining oil.
filling the bottle
put the oil in the bottle and almost fill the bottle using the alcohol, on the remaining space go adding water until the oil almost start going up. THIS IS A VERY HARD TASK, BUT IF NOT WELL DONE THE LAMP WON'T WORK! the alcohol wil get turbid, but after some time it wiil get the original colour. if the oil goes up, put some alcohol.
put the cap on the bottle