Background blocks that will stop foreground blocks being placed? (lava to water martinus machine)

I'm trying to make a martinus trasformation machine on a magma planet but am unsure what blocks I can place on the background to stop Magmarock from forming. Does anyone else know of any?


Also for those unfamilar the Martinus transformation is when an amount of liquid gets super minscule it stops rendering. At this point if it comes into contact with another liquid it'll turn into that liquid and so you can change a liquid from one to another.