The chemistry of these two supposedly interchangeable frits is very similar (the difference being that 3249 has 3% CaO that is missing in F-69). But that does not appear to account for this difference in melt fluidity at cone 04! However, as temperatures increase 3249 rapidly becomes more active. Inspite of the difference here we have found the two work interchangeablely in our G2934Y recipe. Obviously, the F-69 is going to make glazes melt better, at least at low fire.