I have not done this but did a lot of reading about it one time. I remember it was stated to use only the white expanded styrofoam with no additives of any type. The hot metal does burn out the foam and pure styrofoam when it gets hot enough only produces water vapor and CO2 when it burns. Sometimes if the burning is not hot enoguh you do get quite a lot of soot which is just unburnt carbon.
Some foams have fire retardants in them so of course these will not work and the colored foams may have metal compounds for the color so these will not burn away in the same manner.
An open mind is a virtue...so long as all the common sense has not leaked out.