When potters talk, the subject comes up all the time. In an era of insulating brick kilns and high energy prices, why fire long? If you can fire in 6-8 hours, why bother with a 12 hour firing?
Well, if you're like me, you have a file box full of glaze recipes with names attached to the recipes (not "formulas") like "Ferguson's", "Val's", "Shaner's". And, again, if you're like me, you may have started out thinking that the goal in firing those now forty-year-old recipes successfully was in achieving the color promised in the glaze's name (Ferguson's Yellow, Shaner's Red, etc).
Actually, though, the real beauty in those old matte glazes isn't just in the color. The beauty is in the depth of surface.
And here's my hypothesis about long firing times and matte glazes: When these glazes were formulated, most pottery kilns were hard brick kilns -- kilns that were difficult to bring to temperature AND very long in cooling down. VERY long. Those glaze surfaces grew slowly and cooled even more slowly in those hard brick kilns.
In the time passed since those glazes were originally formulated, it's easy to jump to some wrong conclusions about the glaze recipes after having tried unsuccessfully to fire them in the new soft brick kilns. We might wrongly conclude a difference of chemicals from then to now. We might conclude issues of thickness. Those could be factors.
But I've found over the years of firing those matte glazes that perhaps the most important variable to control to achieve those old-time results is TIME. When I fire, I try to mimic the longer firing times required by those old hard brick kilns.