Do I Need More Holes In My Kiln For Ventilation?

[Sumopanda]

Hi There,

I have an e23T L&L kiln.  I have 1/4" holes in the bottom of the kiln. There are 2.  I've been having trouble with bloating and pin holes during glazing.  A colleague suggested I might need to drill a couple of holes in the lid to help with the air circulation.  I read in the L&L manual not to add more holes.  How can I tell if I need more holes or not?

Thanks!

[neilestrick]

What kind of clay are you using? What cone are you firing to? What firing schedule are you using?

[Sumopanda]

I'm using Hagi Porcelain, cone 5 and Cassius, cone 5.  For bisque I'm firing to cone 04 and for glaze I fire to cone 5.  I use a schedule I got from Laguna for bisque and a schedule I got from Digitalfire.com for glaze.

 

1  250°   250°    5 hr

2  100°   700°    1 hr

3  250°  1250°  

4  180°   1850°  

5   300° 1940°    15 min

 

 

 

1   250°   1250  

2   350°   2000  

3     50°   2157    25 min

4   100°   2057    60 min

[neilestrick]

What is Cassius? Another porcelain?

 

Are you getting the problems on both clay bodies? On all glazes, or just certain ones? Commercial glazes, or ones you mix yourself?

 

Lots of variables, we need more info.

 

If porcelain is bloating, it's probably over-fired.

[Mark C.]

I do not think more holes are your issue

[Dick White]

If you are concerned about sufficiency of the ventilation of your kiln, take everything out of the kiln including the shelf on the bottom (which should be raised on 1" posts so air can circulate around the edges of the shelf and out the vent holes in the middle of the bottom). Now, with the vent fan running, hold a lighted match near one of the holes in the base. If the flame is drawn toward the hole, the vent fan is providing sufficient draw through the holes in the bottom. There is generally enough leakage through the peeps and the joints between the sections that additional holes in the top are not necessary. But if, for whatever reasons, the vent fan isn't pulling enough air out the bottom of the kiln to shift the match flame, then the number and size of holes in the top and sides is irrelevant. If there is no suction out the bottom, nothing will come in through the top.

[Pugaboo]

Have you placed orton test cones in the kiln to see how hot you are really firing?

 

Like Neil said, bloating is generally an over firing issue.

 

Pinholes could be several things from too thick a glaze application, or your bisque schedule isn't letting everything burn away, or too fast of a cool down once temperature is reached during your glaze firing which doesn't give the glaze time to smooth out before it drops.

 

Lots of things could be going on and you need to start eliminating some of the variables.

1 clay or both clays doing it?

All glazes doing it on BOTH clays. Only one glaze? Only one Clay etc.

 

First I would check and see just how hot you are actually firing with cones. If too high Program in a cone offset and test fire again with test cones on the shelves to check HEATWORK achieved.

 

T

[Sumopanda]

What is Cassius? Another porcelain?

 

Are you getting the problems on both clay bodies? On all glazes, or just certain ones? Commercial glazes, or ones you mix yourself?

 

Lots of variables, we need more info.

 

If porcelain is bloating, it's probably over-fired.

Cassius Basaltic is an Aardvark Clay. Cone 5.  It's a very dark clay.  When you glaze fire, it turns black.  It's stoneware.  Yes, I have bloating with both Hagi Porcelain and Cassius Stoneware.  Not with B mix though.  

 

The pin holing is from commercial glazes.  Laguna : Forest Green Transparent is the worst, but I also have it come up with other commercial glazes too.

[Sumopanda]

If you are concerned about sufficiency of the ventilation of your kiln, take everything out of the kiln including the shelf on the bottom (which should be raised on 1" posts so air can circulate around the edges of the shelf and out the vent holes in the middle of the bottom). Now, with the vent fan running, hold a lighted match near one of the holes in the base. If the flame is drawn toward the hole, the vent fan is providing sufficient draw through the holes in the bottom. There is generally enough leakage through the peeps and the joints between the sections that additional holes in the top are not necessary. But if, for whatever reasons, the vent fan isn't pulling enough air out the bottom of the kiln to shift the match flame, then the number and size of holes in the top and sides is irrelevant. If there is no suction out the bottom, nothing will come in through the top.

Thanks.  I'll try this out tomorrow and let you know how it goes!

[Sumopanda]

Have you placed orton test cones in the kiln to see how hot you are really firing?

 

Like Neil said, bloating is generally an over firing issue.

 

Pinholes could be several things from too thick a glaze application, or your bisque schedule isn't letting everything burn away, or too fast of a cool down once temperature is reached during your glaze firing which doesn't give the glaze time to smooth out before it drops.

 

Lots of things could be going on and you need to start eliminating some of the variables.

1 clay or both clays doing it?

All glazes doing it on BOTH clays. Only one glaze? Only one Clay etc.

 

First I would check and see just how hot you are actually firing with cones. If too high Program in a cone offset and test fire again with test cones on the shelves to check HEATWORK achieved.

 

T

I have checked with my test cones to make sure I'm not firing too hot.  According to all the glazes and clay specs, I should be okay at cone 5 and that's what the test cones are saying I'm getting to.  (The test cone for 5 is down half way or all the way and the test cone for 6 is just a little bent if any).  I guess that means, I need a slower bisque?  

 

The glazes are definitely not too think.  If anything they are on the thin side.  I thought that might be an issue too.  

 

Yes, there are way too many variables, so I'm having trouble figuring out how to eliminate some of them!  Thoughts?

[Mark C.]

(it turns black.  It's stoneware.)

This and all black bodies can be troublesome -since most are oversaturated with colorants to get that black color. If the Porcelain is bloating you are most likely overfiring it as others have noted.

[Sumopanda]

Just want to clarify, since I'm a relatively new to firing. When people are saying "over fired," does that mean the temperature is too high, or the firing time too long or both?

Thanks!

[Dick White]

Could be either or both. Proper measurement of firing maturity is by using Orton witness cones that bend at a particular amount of heatwork. Heatwork is a combination of time and temperature to determine the penetration of the heat into the ceramic. If one raises the temperature slowly in the last 2 hours of firing, the necessary heat will have fully penetrated the ceramic at a lower final kiln temperature, or if one raises the temperature quickly (assuming the kiln is actually capable of generating enough heat), the final kiln temperature will be higher. Or, one can set a temperature for a lower cone and hold that temperature for a period of time and the heat will continue to penetrate the ceramic, attaining a higher cone outcome at the lower temperature. So, overfiring can be the result of firing to too high a temperature, or too slowly to a lower temperature, or holding too long at a particular temperature.

[oldlady]

is there a  reason you do not use the program built into the kiln by L&L?  have you ever talked to them about the firing of THEIR kiln?  who would know better?

[Sumopanda]

is there a  reason you do not use the program built into the kiln by L&L?  have you ever talked to them about the firing of THEIR kiln?  who would know better?

Tried that first and had lots of problems.  I'll try contacting them as well.  Thanks!

[bciskepottery]

I believe Cassius has high Sulphur content and outgases . . . generally best fired at cone 5, bloats at cone 6.  You likely need to bisque fire Cassius higher to rid it of the Sulphur, etc.  The Sulphur outgassing may be the cause of the pin holing in other glazes. 

[Mark C.]

(it turns black.  It's stoneware.)

This and all black bodies can be troublesome -since most are oversaturated with colorants to get that black color.

These dark clay bodies can be vary troublesome .

[Joseph Fireborn]

You need to figure out what gasses are coming out of the clay, then figure out at what temp those gasses are released. Then during your bisque firing hold at those temps for long periods of time. Also it wouldn't hurt to turn up your vent to pull in more oxygen if you have a vent.

 

When I was working with a dark stoneware I gave up, the bisque wasn't worth the clay body troubles. I was holding for 2-3 hours at certain temps to burn out materials that caused the bloating. Later on even when I figured out the problems there would always be the random pot that would have a single bloating spot on it. Bleh. 

 

If you want a dark body that is really pretty and have 0 bloating issues, you could try SIO-2 Black Ice. However that clay needs to be dried out before using as it contains too much water content out of the bag to throw anything tall. I personally use it as a slip now because I just dislike it so much. I have 150# worth still. 

 

I have 2 holes in my lid. I drilled them because of the bloating issues I had with that black stoneware. They are not a real problem. I just cover one of them up with a kiln shelf post and go about my business while its firing. So if your worried about that I wouldn't be to concerned unless you plan on selling the kiln in the future and are worried that it might decrease sale opportunities.