Summer is here and in Kansas, the rains have finally stopped.  The sun has come out, and the temperatures have SOARED.  As I write this post, we are under a heat advisory with an expected heat index between 103° and 107°F. (yesterday was 114).  How do we tolerate this brutal heat?  Simple…everyone turns on their air conditioning!!! We just had to have Ambrose Air, Inc., come fix our A/C. They did a great job for an affordable price. This can cause some issues with kiln firing, however, but they can usually be mitigated with some simple tricks.  This month’s Tech Tip Tuesday is all about firing during the summer months.

Electricity in the Summer

ohmsLast August, in my blog about elements (click here to read it), I went into a bit of detail about Ohm’s Law.  In  that article I discussed how there are 3 main things that we have to understand when figuring out how a kiln generates heat, and they have a mathematical (and therefore predictable) relationship.  In a nutshell: Incoming Voltage (from your electrician North Brisbane company) is applied to a load or resistor (the element), which then produces energy that can do work (Watts).  In the case of a kiln, the work produced is the amount of heat that is generated per hour.  The mathematical formula is simple

Watts = Volts squared / Ohms.

So in order to determine the wattage, you have to know the incoming voltage AND the resistance of the element.  For simplicity, lets say our resistance is 10 ohms, and our voltage is 240.  In our example:


P = 5760 watts

Converting this to BTU/hr is simple as well.  Just multiply the wattage found by 3.412142…

BTU/hr = P(watts) X 3.412142

BTU/hr = 19653.93792.

So why does this matter?

bucket1As I have mentioned before, firing a kiln is like trying to fill a bucket with water, when the bucket has holes in it.  Not every bit of heat gets retained by the kiln.  Some of it passes through the brick and radiates into the room.  Now this is a GOOD thing, or our kilns would never cool off.  The thing to remember is as the kiln gets hotter, the rate at which the heat leaves the kiln increases, so it takes more heat input to keep the kiln climbing in temperature (albeit at a slower rate).

Secondly, you have to understand that the mathematical equation depends on certain ‘realities’.  The resistance of an element (ohms) is a constant in that you cannot change the resistor without changing the element.  The Wattage is the OUTPUT of the equation.  This means that you cannot change it directly, you have to change the constant or the Input of the equation.  Voltage is the input portion of the equation.  Looking at the math, the relationship between Volts and Watts is direct and positive, and the relationship between Watts and BTUs is also both direct AND positive.   That tells us that as Voltage increases, the BTU/hr output also increases.  If Voltage decreases, the BTU/hr output will also decrease.

Got it!!  Now how does this apply to the Summer?

rtp_comedWe survive the hot summers by using Air Conditioning.  This keeps our living and working spaces comfortable.  Air Conditioners, like electric kilns, run on electricity.  Simply put, there is a finite amount of electricity that the electric company can supply at a given time to your neighborhood.  As everybody’s Air Conditioners come on, there is a greater demand on the electrical grid.  (more people are drawing more electricity out of that finite amount in the grid).  According to the experts of appliance repair in Denver, as the summer days get even hotter, Air conditioners and other electrical appliances, struggle to keep up as the demand on the neighborhood grid quickly exceeds capacity.  What happens then is that the amount of voltage being supplied to your home goes DOWN! My air conditioner went out last summer, I browsed for some on closeupcheck but It was a bad time in my budget to replace it, luckily I was able to find appliance repair near me, and they came over pretty fast.

But ‘thats not all’…the power company’s generators also have to struggle to supply the power to the ALL grids in your area, and generators do not put out a consistent voltage.  There is a normal amount of fluctuation allowed in the line.  Federal Law allows for a fluctuation of ±5% on a 240 volt service (ANSI C84.1).  Normal voltage drop within your home circuit can increase this to a range of -6% to +13%.  This means that your power company is allowed to only supply your home with 225 volts, they don’t have to tell you that it is happening, and its considered NORMAL.

Lets see what that does to the BTU output of our example kiln:

240v – 6% = 225.6 volts (this is voltage after the 6% decrease)

P(w)= (225 ˆ2) / 10


5089.536 x 3.412142 = 17366.219546

(Thats a loss of 2287.718374 BTU/hr from a mere 6% drop in voltage)

Larger kilns (7.5 cu ft and larger) have much tighter tolerances when it comes to the required BTU input.  What that means is as the kiln gets larger, smaller decreases in BTU input have greater impacts on the capabilities of the kiln to hit cone.  Additionally, if your elements are old, and therefore putting out even less heat than normal, then your kiln may not be able to get to ^6 (or sometimes BISQUE) at all!!

So how do we successfully fire in the summer?

In my experience, there are really two main things you can do to continue to fire during the summer months.

#1 – Fire during off-peak electrical hours.

Peak-between-3-7pm-larger-font1The greatest demand for electricity during the summer is during the day.  Once the sun goes down, the temperatures go down a little as well, and our air conditioners don’t struggle as much to cool our houses.  This results in a significant decrease on both the neighborhood grid, AND all of the grids as-a-whole.  Now you have to be careful…firing during Off-Peak means you need to adjust your personal schedule as well.  NEVER leave a firing kiln unattended!!  I suggest starting your firings in such a way that they fire-off in the late evening.  That way, the kiln’s greatest demand for power will come once the neighborhood’s demand for power decreases.

#2 – Reduce the load

overloaded vehicles 2320Firing a kiln on low voltage is like trying to drive a 4-cylinder car through the mountains.  It will likely get there, but it has a much harder time if the mountain is steep, or if there is a LOT of luggage on the luggage rack.  (The height of the mountain is the cone you are trying to hit.  The Steepness of the road is how fast you are trying to fire, and the luggage on top represents how many pots are in the kiln).  Simply slowing down the firing won’t solve the problem, because the shutoff cone is the same.  Taking some of the pots out of the kiln, and therefore reducing the amount of thermal mass, will reduce the needed BTU input to hit the desired cone.  In a real heat-wave, you may need to reduce the load by as much as 25%, and simply fire more loads.  (This concept should get production potters and teachers thinking about changing their production schedule accordingly to acommodate for the increased production time)

#3 – Low Fire in the Summer

Finally, if your work  can be low fire (^04-^06), the summer is a great time to be doing that instead of mid-range or higher.  To keep the mountain analogy from above, its like trying to drive up a much smaller mountain.  Less heat work to do = less BTU required to do it.

Firing in the summer is not impossible, but it can be tricky.  Firing off-peak, reducing the load, and firing to lower cones can mean the difference between continuing to produce during the summer, or having to take a vacation from clay.   The hot months can be a creative opportunity, if you choose to view it that way.  If your normal work and kiln simply can’t do it during the summer, take the opportunity to try something new.  Try and work larger (larger pieces create a lower thermal mass in the kiln)…Try Low-Fire….Take the opportunity to build up on your bisque stock for the Fall shows and orders.  If you are stuck for inspiration, feel free to browse our website for photos, or try one of our Glaze of the Month suggestions.  Above all don’t worry, the temperatures will start to come down before too long, and our kilns won’t have as much trouble getting to cone.  See you in August