by Marc Ward
Clay Times May 2006
Spring has arrived. I get to say that now because I live in the South. But, by the time this article hits your clay-dried hands, it will also be spring in the “Land O’ Salt Trucks”. Spring always brings a blossoming of kiln projects and that has always been my springtime subject. This year is no different.
People want kilns. People want gas-fired kilns. People look at the price of fuel and think, “YIKES”. Building a kiln is an expensive project. If you fire with LP (liquefied propane), one of the most alluring ways to cut your upfront costs is to use a smaller LP tank…. “Hey, I spent all my money on stuff no one told me I was going to haveta’ buy…. A 250 gallon tank is sure a lot cheaper to fill up than a 1000 gallon tank!”
In past columns, I’ve talked about the affects of temperature on LP tanks, bored you with gas physics like Charles’ Law, and basically assumed you’re a math geek. Before we go any farther, let’s repeat the mantra; Math Is Our Friend… Math Is Our Friend… Math Is Our Friend. There,… now you are ready.
It’s really not going to be a whole lot of math. We just need to discuss volume and how it relates to how big your kiln is and what kind of potter you are. And, I don’t mean whether you are a friendly potter or an unfriendly potter. The size of tank you use, many times, depends on whether you are a professional or a hobby potter. Add to this mix; are you a Northern amateur or a Southern pro? How could this affect what size unsightly torpedo you have in your yard?
Professional potters fire a kiln more often than a hobby potter. Because of this, they can’t be encumbered by a tank that doesn’t have enough fuel in it to finish a firing. Professional potters also need to fire when they need to fire. Many times the pro doesn’t have the luxury of waiting till next week to fire. They have to fire NOW. Here is where temperature/location rears its ugly head. The pro in Vermont that has to get ready for the Baltimore Show in February, has concerns the Baltimore bound pro in Florida doesn’t have. Weather…. And I don’t mean snowy roads.
An LP tank, regardless of size, can produces twice as much propane vapor at 20°F than it can at 0°F. 2X as much! At 40°F, we’re talking almost 4 times as much as zero degrees. Because of this, when I recommend tank size to professionals, those above the Mason-Dixon line get a larger recommendation. The amateur can always wait a week or so until the temperature increases some before they fire. They may be able to wait months. Here are the numbers;
I’ve always felt you need at least 4 times the amount of liquid propane in reserve for the amount you are going to use at 60°F. As it gets colder this multiplying factor is going to rise. There are many variables here, so first we need some numbers. (Oh, boy.. I Love Numbers!). There are roughly 92,000 Btu’s in a gallon of propane. And a gallon of propane weighs about 4.5lbs. This stuff is a lot lighter than water at over 8 lbs. per gallon. If that doesn’t confuse you, then let me introduce you to one of the more confusing aspects of LP tank sizing… Some tanks are identified by weight and some are identified by volume (gallons). You can’t confuse the two. A BBQ tank is a 20lb tank. The weight that is expressed is the weight of the fuel only. So a 20lb tank, divided by the weight in pounds per gallon (4.5lb/gal) tells us we have about 4.5 gallons of fuel. A 100lb tank (those five foot high guys that you see at Home Depot) holds about 22 gallons. You can see that confusing a 120 gallon tank (590 lbs) with a 100 lb tank is a major error. So, a 500 gallon tank has a lot of fuel in it, but it may not be enough.
As I said above, there are a bunch of variables involved in this. Because of the complexity of this subject, I can’t go into great detail. If you are unsure of the size of the tank you need, feel free to give me a call… Just remember, my job is to tell people things they don’t want to hear. That said, I’m going to give you some rough guidelines for kiln size, kiln type, and potter type. Again, these are rough guidelines, so if you’re unsure, consult with us or your propane supplier. Also remember, that a raku kiln can get by with a smaller tank because its draw is not constant. You turn the kiln on and off. This gives the tank time to stabilize (warm up some).
Northern amateur, raku potter/trash can kiln:
Stay inside! It’s too damn cold to be outside.
Southern amateur, raku potter/trash can kiln:
Northern pro, raku potter/trash can kiln:
40lb tank (2 ganged together if below 20°F)
Northern amateur, stoneware soft brick 20 cu/ft:
250 gallons (don’t fire below 20°F)
Southern amateur, stoneware soft brick 20 cu/ft
250 gallons (go for it)
Northern & Southern pro, stoneware soft brick 20 cu/ft
(You need a bigger kiln to be a pro)
Northern amateur, stoneware soft brick 40 cu/ft:
250 gallon if you only fire in the summer. You really need a 500 gallon tank.
Southern amateur, stoneware soft brick 40 cu/ft:
250 gallon tank, but don’t let it get too low.
Northern pro, stoneware soft brick 40 cu/ft:
500 gallon tank. You’ll curse the day you didn’t do it.
Southern pro, stoneware soft brick 40 cu/ft:
500 gallon tank. You can get by with the 250, but you aren’t acting like a pro and you’ll get in trouble when you can least afford it.
Northern amateur, stoneware soft brick 70 cu/ft:
What were you thinking?! This kiln is way too big.
Southern amateur, stoneware soft brick 70 cu/ft:
Northern pro, stoneware soft brick 70 cu/ft:
1000 gallon. See Northern pro 40 cu/ft. Same thing applies; 500 gallon will do it but…
Southern pro, stoneware soft brick 70 cu/ft:
500 gallon tank, but don’t let it get too low.
Needless to say, this doesn’t encompass all the variables. You may be using hard brick, ceramic fiber, have a large brick raku kiln, ect. You may live in International Falls or Death Valley. You may travel around doing your work. Just take the time to figure out what you need in the way of LP supply before you commit to the wrong tank size. Bigger may not always be better in many things, but, in tank size, it’s usually true.