by Marc Ward
Clay Times July 2002
Myth # 1…. Propane burns hotter than Natural Gas. Well, this isn’t technically a myth because LP (propane) burns a tiny bit hotter, but the way some people have been taught is that is burns substantially hotter. Sorry, but it ain’t so. LP has a flame temperature of 3573°F while natural gas has a flame temperature of 3525°F. Forty-eight degrees hotter is hardly worth mentioning when we’re talking about thousands of degrees. So, where did this myth come from?
Propane is a much denser fuel than natural gas. A cubic foot of LP vapor has about 2500 BTU’s (British Thermal Units) while natural gas has about 1000 BTU’s per cubic foot. A molecule of LP gas has 3 carbon atoms and 8 hydrogen atoms. The lighter natural gas molecule has 1 carbon atom and 4 hydrogen atoms. Basically, each molecule of LP is heavier than a natural gas molecule. This is why LP orifices in burners are smaller than natural gas orifices…. it takes less volume of LP to equal the BTU’s of a given amount of natural gas. Since it takes two and a half times as much natural gas by volume to produce the same amount of heat as LP, the view that LP burns hotter took hold. Because of it’s denser nature, LP can be somewhat more dangerous. LP has a specific gravity of 1.52, while natural gas has a specific gravity of .64. Great, just what you want… a chemistry lesson. Think of plain ‘ole air as 1. This means that LP is about 50% heavier than air while natural gas is lighter than air. Because of its heaviness, LP can pool in kilns or in low spots on the ground if the air is still.
Myth #2…. You need to candle a glaze load overnight. I’m pretty sure this myth started because of school situations where you might have some less than ideal ware in the kiln. Instead of taking a chance something would blow-up and ruin a bunch of other students work, people proceeded with extreme caution. Some school situations might still need this type of firing, but most don’t. If all the work in the kiln is yours or a combination of potter’s work that have some experience, candling is most likely a waste of time and fuel. “But, Hey!… that’s the way I was taught!” I know, it’s hard to break a ritual. But, in truth, that is what candling is, a ritual that has been handed down from teacher to student. Another thing to consider is the wide variety of ideas about what candling is. I always ask folks what they mean when they tell me they candle overnight…. "What temperature is the kiln when you get to it in the morning?" Answers range from 150 degrees F to 1800 degrees F. Obviously these are different ideas about candling. I would call the former candling and the latter “starting the firing the night before”.
Let’s look at this from a different standpoint. Raku pots are made from stoneware type clays. They are bisqued to the same temperatures as pots destined for a stoneware firing. You might take the raku pot from room temperature to 1850°F in 20 minutes, yet the same type of clay intended for a stoneware firing, with an overnight candle, might spend 20 hours going from room temperature to 1850°F. What’s that about? I believe the 20-hour thing (12 hours overnight, then another 8 hours getting to body reduction) is a waste of time and fuel and does nothing to improve the look or function of the ware. It’s just a learned habit.
If ware has made it successfully through a bisque fire, it’s no longer raw clay with its need to take it easy at first. I started out my pottery life doing the candle thing, but early on decided I didn’t need to candle, quit doing it, and never noticed the difference. Over the years, I’ve recommended to my customers that they give it a try and quit. I’ve never had anyone call back and say that giving up candling for their glaze firing made their ware look any different. Of course, this applies to thin or medium walled pottery, not sculptural work that may be many inches thick. Thick work still needs to go slow to eliminate extreme temperature differences within the piece. Give it try…sleep easier without a kiln in idle all night.