experimental firing fiber downdraft

[CactusPots]

Not being one to leave well enough alone, I'm still trying to learn the nuances of my homemade kiln after 18 years.  It's intrigued me that the bottom shows a hotter temperature after about 1800 or so.  The kiln climbs, although slowly,  in reduction.  So this time, I just let it finish without changes to damper or gas.  Normally, I gradually pull the damper to lessen reduction as the firing moves along.  The bottom finished hotter than the top this time.  I got a dropped 10 on the bottom and only 9 on the top.  Normally, when I shut off, I expect the heat to rise to the top and finish the 10.  Really, there are too many factors at play to have a conclusive answer, I'm just posting this because I think it's unheard of for a firing to finish hotter on the bottom.

36cf fiber downdraft  Propane fired  1 foot thick fiber modules top  stacked with 18"shelves 2 front to back together, 6 inch space between the stacks in line with the flue.  4 venturi burners each side.  Double venturi flue box, pulls from about 200F  no flame at the top of the 4' chimney I have fired the kiln to almost perfect even temperature in the past.  Not being an organized person, I don't take a lot of notes.

Now I really want a oxyprobe.

[neilestrick]

The easiest way to cure a hot bottom is to raise the bag wall. Often just half a brick will do it.

[CactusPots]

Neil,

Bag wall would be the target for a power burner, right?   I don't understand the concept of a bag wall with under kiln vertical venturi burners.  Actually, a subtle design I worked into the kiln is that the burners are very close to the inside side walls.  So the flame, especially at top pressure, drags along the wall to slow it down.

The point of my post was that on my kiln the amount of back pressure in reduction causes the bottom to be hotter.

[neilestrick]

@CactusPots From body reduction on, you should have back pressure out both the top and bottom spy holes. You'll have a lot out of one, but should still have a little out the other. That means that you have pressure throughout the kiln, and it will fire more evenly.

A bag wall is simply a row of bricks on edge between the burners/firebox and the shelves. It forces the flame/heat to go up over the bricks before it can be drawn down towards the flue opening. It typically only needs to be 6-9" tall, although that will vary depending on the kiln. A target brick is used for burners that are going in horizontally, whether venturi or power, and break up the flame so it doesn't just blast against the opposite wall of the kiln.

 

[Bill Kielb]

1 hour ago, CactusPots said:

Neil,

Bag wall would be the target for a power burner, right?   I don't understand the concept of a bag wall with under kiln vertical venturi burners.  Actually, a subtle design I worked into the kiln is that the burners are very close to the inside side walls.  So the flame, especially at top pressure, drags along the wall to slow it down.

The point of my post was that on my kiln the amount of back pressure in reduction causes the bottom to be hotter.

Generally I have found (reduction) can lower the radiant center or flame tips which can cause the bottom to become hotter. The two old updraft Alpines Fire this way like clockwork in reduction. Always begin hotter on top and flip midfire. These kilns have a sort of bag wall (Radiant panel) and horizontal burner which means a target brick about halfway through the flame path. To make matters tougher, they have more mass at the bottom as they are triangular shaped which can readily be observed in the cooldown as the bottom is hotter for many hours.

complicated stuff so could be lots of reasons but I think @neilestrick  point of pressurizing consistently and as much as practical is super important for reduction and keeping unwanted air (oxidation) out. We have oxyprobes and what we consistently are reminded by them and  teach is keep as much pressure out the bottom spy hole as practical to keep the entire kiln in reduction.

The concept of the bag wall is generally to increase the radiant surface and even it out. Target bricks generally split  the flame and because of Coanda effect redirect it again evening things out. Lots of complicated moving energy. Half of a cone would not bother me actually.

[Mark C.]

The reduction is not heating the bottom its more that the flames are there. Neil said what I will say add some heighth to a bag wall. If you hane no bag wall add a small one(not a taget brick) but a sheold to forvce the flame up.

On the oxyprobe subject (I just had two repaired  for $400) now have 4 working ones. Two I bought new decades ago . You can find them used if you look hard enough . I bought two in last two years used -one needed the Platinum  wire repaired -

I also recently got a qoute from a northern European supplier of oxyprobes for under $800 dilivered-I can Pm you that info if you like.

They are great for reduction info but they really are just banother tool that taked the guess work out.

My smnall updraft always has a hot bottom ast the 6 venturies come up thru the floor and thats the hot spot no matter how the falmes go.

In your power burner kiln you shgould just add some deflector (bag wall) to fore=ce heat from getting into load low. One brick height may be enough.

[neilestrick]

40 minutes ago, Bill Kielb said:

 These kilns have a sort of bag wall (Radiant panel) and horizontal burner which means a target brick about halfway through the flame path.

Alpine used a kiln shelf as a bag wall. It allowed them to make the footprint of the kiln narrower because a shelf is a lot thinner than a brick. Resting on top of the shelf and crossing over to the wall were a set of perforated refractory panels, sort of like a grates, that were supposed to break up the flame and help fire more evenly. When I was the manager in the early 2000's I made the call to stop making them becauseI found that they made no difference in how the kiln fired, and the cost of making them was too high. Plus they were a pain to make. It was one of those holdovers from the old days, primarily a marketing gimmick. I also stopped them from using a kiln shelf as a bag wall because it had to be notched into the door jamb and when it warped it cracked out the bricks in the jamb. We instead started using bricks, cut down narrow, instead. Some people found that the kiln fired fine without any bag wall, others needed it. A lot depends on how the kiln is fired, and the size of the kiln.

Another benefit of the bag wall, in addition to forcing the flame upward more, is that it can keep the flame from hitting the ware on the bottom shelf along the fireboxes.

 

[CactusPots]

2 hours ago, Mark C. said:

 

 

The reduction is not heating the bottom its more that the flames are there. Neil said what I will say add some heighth to a bag wall. If you hane no bag wall add a small one(not a taget brick) but a sheold to forvce the flame up.

I'm thinking what the heavy reduction is doing is slowing the exhaust so the heat has a chance to build up in the bottom.  I believe the natural flow in this kiln is up the sides, across the top and down the middle to the exhaust, since the middle is wide open.  I've pretty much proven to my satisfaction that controlling the reduction will give me a perfectly even firing in this kiln.  Anyway, what I have been doing has been perfectly acceptable all along.  I just can't give up tweaking it.

On the oxyprobe subject (I just had two repaired for $400) now have 4 working ones. Two I bought new decades ago . You can find them used if you look hard enough . I bought two in last two years used -one needed the Platinum wire repaired -

I'd love to find a clean oxyprobe for sale used.  I have never seen one offered and it's such a specialized item, I'm not sure where I would even look.  Not that many potters fire with gas anymore, it seems to me.  I wonder if the Euro probes can be serviced like the Bailey's.  I'm looking at $1200  It would have to be mounted in the door, which is 6" thick.  I rarely have luck finding stuff like that.

[liambesaw]

I'd say most production potters still glaze fire with gas.  Hard habit to break 

[Bill Kielb]

1 hour ago, CactusPots said:

The reduction is not heating the bottom its more that the flames are there. Neil said what I will say add some heighth to a bag wall. If you hane no bag wall add a small one(not a taget brick) but a sheold to forvce the flame up.

I'm thinking what the heavy reduction is doing is slowing the exhaust so the heat has a chance to build up in the bottom.  I believe the natural flow in this kiln is up the sides, across the top and down the middle to the exhaust, since the middle is wide open.  I've pretty much proven to my satisfaction that controlling the reduction will give me a perfectly even firing in this kiln.  Anyway, what I have been doing has been perfectly acceptable all along.  I just can't give up tweaking it.

On the oxyprobe subject (I just had two repaired for $400) now have 4 working ones. Two I bought new decades ago . You can find them used if you look hard enough . I bought two in last two years used -one needed the Platinum wire repaired -

I'd love to find a clean oxyprobe for sale used.  I have never seen one offered and it's such a specialized item, I'm not sure where I would even look.  Not that many potters fire with gas anymore, it seems to me.  I wonder if the Euro probes can be serviced like the Bailey's.  I'm looking at $1200  It would have to be mounted in the door, which is 6" thick.  I rarely have luck finding stuff like that.

I purchased oxyprobes from Axner, 800.00 I believe. Probes only, brand new. Beats $1200.00 for a new one. You can order  them to length.

[Mark C.]

I have been inside the probe and theres not much to them-Platinum wire which is really costly -sold nby the inch and a termainal block and regular wire leads with plug ends

thats it so simple. And the meter which is exactly like say an ohm meter. Axner did my repairs (Glen does that work there)

I found one of those used ones at potters web under equipment and the other at the old facebook potters sale site  now gone but replaced .(now there are two of those sites)

Used ones are about 500$-they may need wire repair as shipping one is fragile and requires great packing .

The European one is what I would get. These meters went thru the roof price price wise last decade.I never posted the link here as most here fire with electrics-I found them last fall .

All the production potters I personally know fire with gas .That is folks in California,Oregon ,Washington,Arizona, New Mexico,Utah,Idaho, Wyoming-those cover all the states I know potters in personally-oh ya Alaska as well.

I know very few production potters firing cone 6 electrics for living  (this has been covered a lot here before.) I know of two-ones in Texas the other in AZ.

[CactusPots]

Well, it didn't turn out as uneven as I thought it might.  My theory of turning it off with the bottom having more work than the top is correct.  The top of the kiln, insulated with 12" of fiber modules will continue to work.