19 May 2015

Venting Your Kiln; Why & When?

When I first started fusing glass in 2001, the subject of venting your kiln was really not much of a discussion. Everyone vented the kiln to cool it. The kiln manufacturers would wince when the topic came up. They hated it, but we all thought that was what we were supposed to do. 

In glass fusing we called it "crashing" or crash cooling. We basically opened the kiln at 1470F (full fusing temperature) and then let cool air into the kiln until the thermocouple reading was below 1200F. This might have to be done a few times to account for the temperature bounce back when we closed the lid. The entire process was done to avoid devitrification on the glass. The result was the eventual cracking of kiln bricks and lids. This process of this sudden venting was terrible for your kiln, and didn't always eliminate the devitrification problem, if you remembered to do it at all. If you weren't careful, many a burned hand or singed eyebrow was the result.

The next type of venting was what ceramics folks did in their kilns to draw the metal oxides out of their kilns and bring in oxygen. This venting, usually  a downdraft or vent hood "system" provided a good air exchange in the kiln and led to bright colors in glazes. A "Vent Master" was and is a necessity for firing ceramic glazes at higher temperatures. I really didn't fully understand it fully until years later.

Things have changed a lot in the last 15 years. We now take better care of our kilns and keep them much cleaner, and crash cooling is thing of the past. We anneal at lower temperatures, and we know when we should vent for good air exchange! We handle devitrification with overspray, sandblasting, and good old kiln maintenance. It's just a smarter world now! ...But we still need to vent if we want our colors bright when we paint, print, or decal!

Here's the WHY! Harry Sowersby from Creative Ceramics, the folks who make ColorLine paints and screen printing paste, explained it to me this way, "...during firing the burning off of organic binders in paints/enamels/glues/etc creates a reduction atmosphere inside of the kiln, meaning that there is less oxygen available. Bright colors like reds, oranges, and yellows, (selenium based colors) require an oxygen rich environment to maintain their color integrity. So it's necessary to vent the kiln to bring in fresh oxygen throughout the entire firing process until they reach peak temperatures." Once the burnout is complete, the vents can be closed. Now I totally understand that!

So what do you do it you don't have a vent? 

Some studios leave the kiln door ajar when firing anything with organic binders or other burnout items. 

Some studios raise their clamshell kiln lids with 1/2" kiln posts to vent.

Some studios pull all the plugs out from their kiln vent holes. (Those are not peep holes for your eyeballs, in case you didn't know.)

Some folks are even going so far as to vent when burning out 1/32" kiln paper a.k.a. Thinfire paper or Papyrus sheets. They say it reduces the chances of devitrification and keeps colors brighter. If you have a thought on this or an opinion. Let us know what you think!  

To Vent or Not to Vent.... When is the Question!

Your Kiln Gal,
Gail