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48Cuf Gas Kiln Help


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If my numbers are close, that's about 43 gal of propane, where LP gas weighs 4.1 lb/gal. that might be a bit high but with some more firings, you might get it down 10 or 15%.

I pay about $1.60/gal and you about $2.20/gal just close not an exact amt.

Wyndham

 

My math might be way off, I'm not sure I have the numbers right

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For reduction we normally, during and at the end of the firing after soaking block partially the chiminey with bricks at the top and lower the primary air intake.

 

As for the Fuel we use LPG. We are based out of india, and the gas provider is probably the last person who is able to help us.

Also the guy who made the burners for us is postponing his visit to check them out and not showing any interest in it at all.

 

As far as it goes, the burners are identical to the ones fitted to our old 27cuf kiln. same nipples/orifices and all.

Afther telling him that our new kiln was 36cuf he said the nipples/orifices are fine being the same as for the old kiln.

 

I made a couple of calculations and estimated as per the Kiln book ref. the BTU imput required to be:

 

14K BTU per cuf. or kiln X 36cuf= 504KBTu

Now if we divide this number by 4 burners we will get a result of 126KBtu per burner.

This should give an orifice size of 3mm diameter.

 

The ones we have now are nearly half this size. lets call it 2mm.

....

 

How did you arrive at 3 mm?

I searched around a bit and I found out that 3 mm (0.11811") is going to give you close to 105K BTU , so you are about 21 BTU's short per burner,  or 84 BTU in total.

 

 

EDIT: and if you are "reducing", you need to give it more air from intake.

Can you make the burner holes bigger, so more secondary air gets in?

 

PS! Do not take my word for it, I am not a pro nor even a noob :)

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Hi marcia,

 

the efficiency of the two single large burners stops at 3.5 psi  (I could fit a .2mm smaller orifice though and the increased velocity of the gas at the same pressure should be able to drag in more air. 

The larger burners primary air is regulated by a dial/disc and the secondary is pretty big at a square 3" in side size.

 

 

I can't say as I am not sure of the kiln insulation, but it does seem like a lot of gas after i did the conversion.
170 pounds of propane. Anyone out there firing a kiln this size with propane care to comment?

It sounds like your burners were putting out too much gas. see if you can use the larger ones alone and have them on a harder flame. can you add more air intake on them by turning a disc at the rear of the burner?


Marcia

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Hi Mart,

 

Having a look at the orifices the other day, I discovered the old ones in the small burners were 1.2mm dia.

The other issue is the pressure... the more pressure you put in the line, the more gas will come out, and with a LPG cylinder you can get up to 25 PSI.

But the burner size and design as well as pressure and orifice are all influential to a good flame with enough btu's.

 

Anyways... the big burners with a 1.8mm hole seamed to do well up to 3.5psi after which the flame quality deteriorated...

I'm now curious to print out a chart I've found on the net for different drill size and output of BTU's at different pressures and check with smaller size orifice at higher pressure.

It could well be that my burner is not able to generate enough venturi effect and is therefore not sucking in enough primary air to mix in the fuel.

Apart from that otherwise the kiln is quite responsive and I don't think there is a circulation problem... i.e. if the kiln is not reducing and I blast the gas though causing reduction, it takes less then 3 seconds for the unburned green gas flame to come out of the chiminey...

 

FOR your info, attached the PDF with orifice sizes which helped me to find out the burner tip size and orifice size...

 

 

 

 

 

 

 

 

For reduction we normally, during and at the end of the firing after soaking block partially the chiminey with bricks at the top and lower the primary air intake.

 

As for the Fuel we use LPG. We are based out of india, and the gas provider is probably the last person who is able to help us.

Also the guy who made the burners for us is postponing his visit to check them out and not showing any interest in it at all.

 

As far as it goes, the burners are identical to the ones fitted to our old 27cuf kiln. same nipples/orifices and all.

Afther telling him that our new kiln was 36cuf he said the nipples/orifices are fine being the same as for the old kiln.

 

I made a couple of calculations and estimated as per the Kiln book ref. the BTU imput required to be:

 

14K BTU per cuf. or kiln X 36cuf= 504KBTu

Now if we divide this number by 4 burners we will get a result of 126KBtu per burner.

This should give an orifice size of 3mm diameter.

 

The ones we have now are nearly half this size. lets call it 2mm.

....

 

How did you arrive at 3 mm?

I searched around a bit and I found out that 3 mm (0.11811") is going to give you close to 105K BTU , so you are about 21 BTU's short per burner,  or 84 BTU in total.

 

 

EDIT: and if you are "reducing", you need to give it more air from intake.

Can you make the burner holes bigger, so more secondary air gets in?

 

PS! Do not take my word for it, I am not a pro nor even a noob :)

 

 

Gas Orifice Capacity Chart.pdf

cat0454.pdf

Gas Orifice Capacity Chart.pdf

cat0454.pdf

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Do you remember that Swiss guy Daniel Bernoulli, with a huge wig, who stated that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Point is, that not only more gas comes out, but the gas comes out at faster speed. That is why at some pressure/nozzle orifice size it chokes and lots of gas is wasted.

1 kg of propane has energy 50.3 MJ (megajoules) or 47700 BTU and to burn 1 kg of propane you need 3630 g of O2.
Our atmosphere (air) has only 20.946% of O2 so you need about 17.3 kg of that stuff entering your burner/kiln in same time frame as propane.

If secondary air is not getting in , reduction happens (gas is wasted).

Now all we need is the nozzle_orifice/gas_pressure/burner_opening ratio. :)

Thankfully someone has done most of the hard work by calculating nozzle orifice/pressure to BTU output but we also need a kiln port?(opening for the burner) area calculation for the air.

I bet that this kiln book has it somewhere written down. If it has, can you please post it ?

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I guess that resolves the previous question :)

 

A rough rule-of-thumb ratio is one square inch of flue area to 8,000 BTU’s of maximum gas input

 

 

If area of flue opening (inside) is usually close to or equal to area of burner port opening .... 

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Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill

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on the previous page,

"...if the kiln is under constant reduction and will not climb in temperature easily, the portholes can be widened to provide more secondary air for combustion. at the same time make sure that the exit flue is not too small. However, most of the time the burners will have oversixed orifices, thus throwing off the air/gas ratio for proper combustion.To solve this problem, replace the orifices with smaller ones. (Normal orifices for low pressure burners are between ..... 38 and 42 for propane"

 

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If I go by this  and consider the kiln to require 12K BTU per CUF   it will come to 36x12/8=54 SQ INCH   my flue size now is 7"x4.5"= 31.5  SQ INCH

I don't think an increase in flue size by  40% more or less is the solution...  especially if the burners are homemade ... I consider it to be more of a orifice size/ air inlet problem.

But this said, I am planning to remove 2 bricks and add 2 1/2 bricks to the flue gate, thus increasing the flue size by 13.5sq" ... to 45SQ"  and block off part of it to make it roughly 37-38 sq".

 

 

 

 

 

A rough rule-of-thumb ratio is one square inch of flue area to 8,000 BTU’s of maximum gas input

 

 

If area of flue opening (inside) is usually close to or equal to area of burner port opening .... 

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For gas kilns in The Olsen book it states that the exit flue port should be equal to the area of the inlet ports sizes.

 

On rule of thumb NB 2 it would come out to 90Sq".... 3 times more then what i have now...

 

By rule 3 it should come to 63 sq "  ( double)

 

An by rule 1 which is on the book ( 2 large square ports at 3"x3" + 2 round ports at 2"1/2 dia  =   34sq")

 

 

 

 

 

Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill

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In bernoulli's principle the talk about "potential energy" is in terms of pressure not BTU's....

 

If the PSI on the pressure regulator at the cylinder is the same both for a, lets say, 1.6mm diameter hole or a 1.8 dia hole, the gas quantity coming out of the orifices might be the same, only the speed at which the gas travels will be different.

if the gas travels faster it should create more venturi effect ...

As far as secondary air, the inlets are quite big,,,

 

If you do a test with the burners out of the kiln it is quite easy to determine the flame quality and to see if enough primary air is there or not...

This is mostly due to burner design....  i.e. I can have a perfect burner design, but if the orifice is way over or under sized, it will work at a % of its real efficiency.

 

I will have to do an empirical test...  and go down from 1.8mm dia at 3.5psi to a 1.6 and maybe 1.4 at a slightly increased pressure and see if there is too much turbolence or if the flame burns better...

 

Do you remember that Swiss guy Daniel Bernoulli, with a huge wig, who stated that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Point is, that not only more gas comes out, but the gas comes out at faster speed. That is why at some pressure/nozzle orifice size it chokes and lots of gas is wasted.

1 kg of propane has energy 50.3 MJ (megajoules) or 47700 BTU and to burn 1 kg of propane you need 3630 g of O2.
Our atmosphere (air) has only 20.946% of O2 so you need about 17.3 kg of that stuff entering your burner/kiln in same time frame as propane.

If secondary air is not getting in , reduction happens (gas is wasted).

Now all we need is the nozzle_orifice/gas_pressure/burner_opening ratio. :)

Thankfully someone has done most of the hard work by calculating nozzle orifice/pressure to BTU output but we also need a kiln port?(opening for the burner) area calculation for the air.

I bet that this kiln book has it somewhere written down. If it has, can you please post it ?

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For gas kilns in The Olsen book it states that the exit flue port should be equal to the area of the inlet ports sizes.

 

On rule of thumb NB 2 it would come out to 90Sq".... 3 times more then what i have now...

 

By rule 3 it should come to 63 sq "  ( double)

 

An by rule 1 which is on the book ( 2 large square ports at 3"x3" + 2 round ports at 2"1/2 dia  =   34sq")

 

 

Gianda,

2 ports at 3"x 3"= 18 sq in + 2 round ports at 2 1/2" d. (Pi(3.14) times Radius squared) 1.25"X 1.25"=4.9" sq in" so 9.8" + 18" =27.8" should be your exit flue size. That indicates me ( kinda working backwards here) that it is likely that your inlet ports are too small. Have you checked the table in the book that give recommended sizes of inlet ports for a given size burner nozzle? If not, do so , and then adjust your inlet and exit flues accordingly.

You should be able to fire to temp easily in oxidation, if you can't then it's probably  the kiln being choked at inlets or exits or both. I sure am curious to see what the problem actually turns out to be. Please let us know when you figure it out. Bill

 

 

Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill

 

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Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

 

Your answer

 

----

 

and not:

 

Your answer

 

quoted text

 

 

 

Cheers!

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Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

 

Your answer

 

----

 

and not:

 

 

Your answer

 

quoted text

 

 

 

Cheers!

Didn't know there was a rule. Seen others do it, so I figured I'd just keep my reply next to what i was responding to. Sorry to have annoyed you so.

Hope you have a good evening even though.

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You know Mart,

You will get more flies with honey....

And you'll be more likely to get someone to follow your top quoting preferences if you reword your instructions.

I find your remark "extremely" objectionable and annoyingly off topic. 

You added nothing to solving the problem at hand as this is not an internet ettiquette discussion forum nor topic.

 

 

Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

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The burner ports and flue openings must be sized properly, and the methods listed thus far work well. However secondary air is also affected by the draw of the exit flue as well. If the chimney is too short, there won't be enough draw to bring in the secondary air or do an effective job exhausting air in general. How tall is the chimney?

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Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

You know Mart,

You will get more flies with honey....

And you'll be more likely to get someone to follow your top quoting preferences if you reword your instructions.

I find your remark "extremely" objectionable and annoyingly off topic.

You added nothing to solving the problem at hand as this is not an internet ettiquette discussion forum nor topic.

 

FTFY (Fixed That For You)

I'll use larger smileys next time, when drawing attention to a simple, logical and also useful net etiquette.

 

I agree, this is off-topic so lets get back to the burner port holes.

 

Working back from the exit flue size via kiln capacity in cubic shoes... toes... no.. feet, gives very different results as shown before.

 

1. Add the total inlet flue areas (ports) to find the exit flue area.

 

Start from 4. and 3.

 

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

 

3.42*2.5=8,55inc2 - 55.16 cm2(divide that by number of burners and I get 55.16/2=27.58 and sq2 from 27.58=5.25x5.25 square opening or circular opening diameter 59,2 mm.

 

I can not fit my burner in to that burner port because it's outside diameter is 60 mm

 

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

 

54720/7000=7,82 - 50,44 cm2 is smaller than 1 and will not work for burner port.

 

Finding the BTU's one needs (from another book)

4. for cone 10, 6-8 h firing) 16,000*kiln size in cf 3,42*54720=54720

 

 

Even this will be too small for the burner port:

If I am going to take the result from 3 2 (27.58) and add it to the area of my burner (28.27). This will give me a space around the burner with area 27.58.

Final diameter for the burner port will be: 28.27+27.58= 55.85 cm2 or a circular hole with a diameter of 8.43 cm

 

Sounds like those equations work for larger kilns an not small, less that 5 feet3

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The math for computing the inlet and outlet size is the same regardless of the size of the kiln, whether 10 cubic feet or 100. Yes, there is a point at which the burner ports cannot be smaller than a certain size, but having ports that are a little oversized won't affect things all that much. That's why there's a damper on the flue. Problems only arise when the ports are too small, or way too large.

 

I've read this thread several times, but I still have a lot of questions. Many photos of the setup, inside and out, would really help a lot!

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Thanks guys, for all the support and help...

 

As said in my last post, problem has been solved... :)

 

Overall, I think that i was a combination of problems... Undersized burners to start with, burner ports to continue, exit flues maybe, and during the last firing, after finishing the gas on the two small burners, the temp started rising like crazy on the two new larger burners alone  from 1184 to 1215 in 8 minutes... ( 1 deg, C every 15 seconds. which at high temps is pretty good as far as my experience goes.) The final temp reached, after the gas on the larger burners run out too was 1235. which was reached in 12 minutes or so...

 

Looking at this, I feel pretty confident that I should be able to get to cone 10 at the next firing...

 

Here a sum up of the changes done for ref.

 

2 Large burners using 35mm iD SS pipe with 60mm dia burner head and 1.8mm orifice. Burner ports of 75x75mm square.

2 small burners using  25mm iD ss pipe with 45mm dia burner head and 1.2mm orifice. Burner ports of 60mm dia.

Exit flue is 7"x4.5"

Kiln size is 3'x3'x4'depth chamber.

Chimney height is 5'

 

I can post pics of the kiln for further reference, but as of now, after building my first kiln,  if I had time I would rip down everything but the chimney and floor and rebuild it from scratch... ( well... lots learned from building this first one) and waiting for the next one...

 

Also here i India materials are really cheap and saved a lot of $ by building my own kiln instead of asking someone to do it for me... ( here there are not many real professional companies that build them anyways...) All in all, my time not counting in the equation I have spent to build the kiln 80,000INR which in $ is about 1330$...

 

Carborundum shelves 1'x2'x1" come at roughly 40$

One 7 meter roll of high temp. 1470 deg C fiber costs about 35$

High alumina bricks 0.56$ a piece

Iron comes at little less then 1$/KG so the frame including welding and all set me back 250$

 

 

ANYWAYS, one never stops learning!

 

here the pic of the large venturi built from SS off the shelf parts for 15$ each...

 

 

 

 

And here some pics of the pots that came out of the firing @ 1235Deg C.  some glazes melted really well... some others not...

Some others might have needed a thicker application....

P.s. the pottery went through 2 firings !!!  hence the cracks on some of them... first firing @ 1125  and second firing at 1235.

 

 

post-58506-0-90062200-1376917682_thumb.jpg

post-58506-0-84305700-1376917705_thumb.jpg

post-58506-0-07801300-1376917725_thumb.jpg

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post-58506-0-90062200-1376917682_thumb.jpg

post-58506-0-84305700-1376917705_thumb.jpg

post-58506-0-07801300-1376917725_thumb.jpg

post-58506-0-81465100-1376917746_thumb.jpg

post-58506-0-24617800-1376917764_thumb.jpg

post-58506-0-69557200-1376917785_thumb.jpg

post-58506-0-83533500-1376917807_thumb.jpg

post-58506-0-17551200-1376917824_thumb.jpg

post-58506-0-09472200-1376917843_thumb.jpg

post-58506-0-32125300-1376917868_thumb.jpg

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Thanks guys, for all the support and help...

 

As said in my last post, problem has been solved... :)

 

Overall, I think that i was a combination of problems... Undersized burners to start with, burner ports to continue, exit flues maybe, and during the last firing, after finishing the gas on the two small burners, the temp started rising like crazy on the two new larger burners alone  from 1184 to 1215 in 8 minutes... ( 1 deg, C every 15 seconds. which at high temps is pretty good as far as my experience goes.) The final temp reached, after the gas on the larger burners run out too was 1235. which was reached in 12 minutes or so...

 

 

 

I have highlighted a small piece of text that is worth noting because I think it exactly pinpoints the problem. After the 2 small burners were shut off the kiln took off. If you made no other adjustments besides shutting off the 2 small burners I think that they are not mixing in enough air.

I'd try a firing using only the two newer burners and see how even you can get the temperature. If that doesn't work replace the 2 older ones with burners like the other two new ones and give another try.

 

You don't want the temp to rise that quickly. You are trying to do a specific amount of heat work on the pots/glazes in the kiln and time is a component of heat work. If the temp rises too fast, the glazes may not melt completely and the clay may not become totaly mature

 

 

The pots are great BTW. The brushwork is fantastic.

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Thanks guys, for all the support and help...

 

As said in my last post, problem has been solved... :)

 

Overall, I think that i was a combination of problems... Undersized burners to start with, burner ports to continue, exit flues maybe, and during the last firing, after finishing the gas on the two small burners, the temp started rising like crazy on the two new larger burners alone  from 1184 to 1215 in 8 minutes... ( 1 deg, C every 15 seconds. which at high temps is pretty good as far as my experience goes.) The final temp reached, after the gas on the larger burners run out too was 1235. which was reached in 12 minutes or so...

 

 

 

I have highlighted a small piece of text that is worth noting because I think it exactly pinpoints the problem. After the 2 small burners were shut off the kiln took off. If you made no other adjustments besides shutting off the 2 small burners I think that they are not mixing in enough air.

I'd try a firing using only the two newer burners and see how even you can get the temperature. If that doesn't work replace the 2 older ones with burners like the other two new ones and give another try.

 

You don't want the temp to rise that quickly. You are trying to do a specific amount of heat work on the pots/glazes in the kiln and time is a component of heat work. If the temp rises too fast, the glazes may not melt completely and the clay may not become totaly mature

 

 

The pots are great BTW. The brushwork is fantastic.

 

 

Hi Ben,

 

thanks for the compliments, well, the 4 small old burners wouldn't reach temperature, while the two large ones seam to be doing the work just fine...

What I'll try next firing is to use the small burners alone for the first 4 hrs because at low pressures they seem to be doing just fine and the temp rises nicely and steadily... for the last 4 hrs ( 600deg onwards,) I'll shut those 2 off and replace them firstly with 1 single large new burner and then add an extra one when the temperature raise seems to have slowed down a little.

Hopefully it'll work!

 

P.s.  When the 2 small burners went off, I plugged the holes and left only the two single burners to do the work.

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