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Pugaboo

How Much Of A Gap Between A Piece And Shelf Needed?

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Pugaboo    438

Still learning lots and have another question...

How much of a gap between the top of a piece and the bottom of the shelf do I need to leave?

Have heard/read squeeze it in as tight as possible all the way up to an inch because the pieces swell during firing so I am a bit confused.

 

Terry

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Mark C.    1,808

The clay will shrink in a glaze fire and a bisque fire if it keeps getting fired hotter.

So with that you only need it to clear the shelve the thickeness of a piece of paper.

Now if its a refire glaze piece than it can expand a bit with heat so its need more room.

Mark

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TJR    359

I agree with Mark. You want to cram as much pottery as possible into your kiln. If it is a bisque, and since there is no glaze, pieces can be stacked on top of each other. For a glaze fire, you can stack right up to the lid. Use a yard stick across the top to make sure work is not higher than the kiln. Do not leave the yard stick in the kiln!

TJR.

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Pugaboo    438

What if the piece has underglaze or RIO on it? Will either of those things make a piece stick to stuff?

 

Terry

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Benzine    611

Underglaze won't stick, though it can leave some colored marks on the shelves and other pieces if you stack them.

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Mart    23

I agree with Mark. You want to cram as much pottery as possible into your kiln. If it is a bisque, and since there is no glaze, pieces can be stacked on top of each other. For a glaze fire, you can stack right up to the lid. Use a yard stick across the top to make sure work is not higher than the kiln. Do not leave the yard stick in the kiln!

TJR.

 

Why?

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oldlady    1,323

so it heats evenly???  because the yardstick will burn up???  :rolleyes:

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neilestrick    1,381

The problem is during the glaze firing. Glazes bubble up as they melt, then smooth out. Too close to the shelf and they'll touch as they bubble. Give at least 1/8 inch. 1/4 to be safe.

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TJR    359

 

I agree with Mark. You want to cram as much pottery as possible into your kiln. If it is a bisque, and since there is no glaze, pieces can be stacked on top of each other. For a glaze fire, you can stack right up to the lid. Use a yard stick across the top to make sure work is not higher than the kiln. Do not leave the yard stick in the kiln!

TJR.

 

Why?

 

Because there is labour involved. It takes time to fire a kiln. The more you have in there the more product you produce. There are also energy costs involved with firing a kiln. You have to fire it up to the same temperature wether you have one mug in it or 83.Do you not pay for your firings?

T.

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erinwells    0

Wer... while I do agree that you can generally cram as much stuff as possible into a bisque, you generally do want to leave an inch or so between glazed pieces and shelves during a glaze firing.  It is true that overall, during both the glaze and bisque firing process the pieces do shrink, there are other factors that encourage us to leave a little room. You have a good shot and reducing a lot of cracking, dunting, and crazing if you leave adequate space for air to flow above your work during the firing as well as reducing the risk of a piece warping/bending and getting stuck to the bottom of the shelf above it. (In high fire some of my works have gone from bowl too "taco" and fused onto shelves above them) It also means you will have a more even firing! Generally speaking you will see more issues with larger pieces that do not have adequate space that will want to crack or warp if they don't have room...

 

I generally leave 3/4 inch room above the tallest piece on the shelf and, about a half-inch between pieces on the same shelf. 

 

PS, You are right that pieces do expand, but it is only for a little while near quartz inversion and is pretty negligible as the work has already decreased in size significantly. 

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OffCenter    82

You have a good shot and reducing a lot of cracking, dunting, and crazing if you leave adequate space for air to flow above your work during the firing ...

 

How so?

 

Leaving an inch is a waste of space. Neil's suggested spacing is what I go with when I need to pack a tight kiln.

 

Jim

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TJR    359

Newbie;

As I have stated twice previously,you do not need to leave a large amount of space between your work. If your pieces are dunting and warping and turning into tacos, you have serious problems with your clay body in that you are firing it to too high a temperature. When loading a glaze kiln, you put the piece in so that it touches it's neighbour and then you move it away slightly. No more than 1/4 of an inch. No glaze pieces should touch. If you are firing a wood kiln or a salt kiln and you are wanting atmospheric effects, you can leave more space. For gas or electric, there is no need to squander space. Have you taken a look at Mark Cortnoy's pictures of his stacks? I don't think you could get a fettling knife blade between pots.

In these trying times of high energy costs, why are you firing air? Lets use our kiln space efficiently, and walk softly on the back of Mother Earth.

There! Now I have said it. After three times I feel that I have thoroughly explained my position.

TJR.

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perkolator    54

really depends on the size of work and whether or not it's been fired previously.  if it's pottery and stuff shorter than say 24", then you can get away with a very small space like 1/8-1/4".  if it's large-scale sculpture with lots of mass then this is where i'd have at least 1/2" depending on the size and clay body if firing greenware (my students are loading 2-6ft sculptures though).  when clay heats up it goes through quartz inversion and the molecules can expand 1-2%.  this is why you've been told to leave a space.  my general rule of thumb for most small items is if you can stick a piece of paper or cardstock between the objects then you're good to go!

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 How does heat-work tally up? If you fire and empty kiln to a temperature (or cone), and fire it again to the same endpoint full of wares, how much percentage wise more power is used? I think I'm having calibration issues with my sitter or I'd test instead of ask, but it seems to me, over my three firings so far, that the one full load took 400% longer to fire. I'm really unsure about the sitter though...

 

(Sorry if this is off-topic, but several posters mentioned filling the kiln to save energy.)

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Chris Campbell    1,088

Disclaimer .... i am not a firing genius ...

What I think you are asking is how much heat work is affected by mass. How much longer does a full kiln take over a partial one?

I used to stack my kilns like a squirrel hoards nuts ... It would take a lot of time to load and unload.

But now, I make larger pieces in weird shapes so have more air spaces.

I cannot say that it takes that much longer ... Certainly not hours longer.

Any other opinions or different results? My firing profiles have changed over the years so that might be what keeps it the same??

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bciskepottery    925

I generally leave between 1/4 and 1/2 inch between the top of the wares and the next shelf, both bisque and glaze firings in an electric kiln.  Clay both expands and contracts during a firing, so the wares need some breathing room.  Plus, for glaze firings, you need to be aware of your glaze properties.  Some glazes spit and can contaminate wares next to them.  Some glazes flash fumes (copper glazes); so you don't want them fuming on a white vase or on one you decorated for hours with underglaze and then glazed in clear.  Sometimes fuming is nice . . . but not always.  And as Neil noted, glazes boil and bubble. 

 

Packing a full kiln is good practice, but allow some room for heat (electric kiln) and/or flames (gas or wood kiln) to move among the wares.  Especially on fuel kilns, that flame whipping/winding through the wares on the shelves creates beautiful effects.  Packed to closely, you end up blocking the flames. 

 

I don't recommend touching wares and then backing off to space them . . . it's a good way to leave a deposit of your favorite iron red glaze on a celadon vase.

 

Kiln loading (like most things in pottery) can be an art unto itself . . . you really get better, more efficient, and more knowledgeable the more you do it.

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oldlady    1,323

the above post was typed faster than mine and so it might appear that i read it before starting this.  not true.

 

all kinds of info here so i guess it is ok to add another opinion or two.  i have always used an electric kiln.  i pack the stuff in, using up to 9 full shelves per firing.  it all works fine.  anything that i don't like was something stupid that i did, not the kiln, clay or glaze. it is not packed paper tight, maybe pencil size spacing.  i do not have a kiln vent.  i close it all up after about 1000 degrees or so.  the heat is radiating inward from the heating coils inside the bricks.  radiant heat must have heat waves.  any info that is useful re how the waves travel?

 

in 1979 or so, the pres of the kiln club in wash dc did something bad to her leg and i loaded her gas kiln for her. it was the same size and shape as a large electric kiln, not a catenary arch or some such so i wasn't afraid to load it.  she made all kinds and sizes of stuff.  i loaded it full and stacked things on half shelves that hung over other work and made it all fit.  she said it was one and a half times more than she would have fit in the kiln and  the best firing she had done because the flame had to travel all around before exiting. that is what happens in gas and flames and air move during the firing.

 

the original poster has a small electric kiln with 15 inch half shelves.  this was her second attempt at firing and it was a bisque load.  most pieces were about the same size but there was one really tall piece she was trying to fit in and around. i know she did not mention all this in the post. she has an instruction manual from the kiln manufacturer and all kinds of "advice" from other sources.  

 

she does have a kiln vent so maybe air does move around inside HER kiln.  as far as i know, (agreed it isn't much)  nobody has climbed inside an electric kiln and watched how the air moves during a firing.  that info might be helpful, any volunteers? short of that, how does anybody know anything about this stuff?

 

there is a lot on info out there today and some of it belongs in chris campbell's post about the worst thing you ever heard. it is hard to know what is right in each situation.   what works for me might not for someone else. don't be afraid, newbies, it will some day become as familiar as your toaster. 

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Mark C.    1,808

This is what works for me as far as  glaze loads-I have mainly two size (nerds) shelve spacers which add a little to the stilt height . Mine are 5/8 and 3/4 inch small pieces of silicon carbide or mullite shevles. I have a few 1/4 ones but they die quickly due to fuzing no matter how much I coat them.

This glaze fire was just loaded a few minutes ago-my Third since the fourth of July -my 16th this year in my  35cub.car kiln. I have also done 12 small kilns this year in a 12 cubic updraft (firing now) and this loading is what works for me the past eons

This particular load is a bit odd as some of the forms are not my usual and take more wasted space (cannisters and a large 18 inch bowl)

This load has 36 12x24 inch advancers and is going to be fired in am to cone 11- 1/2 way down gas reduction fire in AM.

I place the pots so they almost touch and have forever with no sticking to one another. Refires will expand more and need more space as you can see they are a few in this load. As far as top space as close as I can get a shelve to clear with my nerds is what works for me all these years.

For those of us who make a living at this more in each kiln load is a key point. This may not work for everyone but it does for me which is what counts. My glazes do not expand more than almost touching the next item/shelve.

Every few years I will stick a pot to another-usually because I have shoved them together after loading them by moving them after placing them the 1st time around around. When you load a kiln like this many times all this is second nature-how tall of a stilt is needed what is the hieght of wares etc.This kiln loads in about 1 to two hours depending on how much small stuff is in it-I'm not a numbers guy counting pots but there is over 400 pieces in this load I'm sure.

Mark

 

 

post-8914-0-95860200-1374548850_thumb.jpg

post-8914-0-95860200-1374548850_thumb.jpg

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oldlady    1,323

i knew you worked hard, mark, that is crazy!  you make more pots in six months than i have in a lifetime. maybe more on one shelf.

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TJR    359

This is what works for me as far as  glaze loads-I have mainly two size (nerds) shelve spacers which add a little to the stilt height . Mine are 5/8 and 3/4 inch small pieces of silicon carbide or mullite shevles. I have a few 1/4 ones but they die quickly due to fuzing no matter how much I coat them.

This glaze fire was just loaded a few minutes ago-my Third since the fourth of July -my 16th this year in my  35cub.car kiln. I have also done 12 small kilns this year in a 12 cubic updraft (firing now) and this loading is what works for me the past eons

This particular load is a bit odd as some of the forms are not my usual and take more wasted space (cannisters and a large 18 inch bowl)

This load has 36 12x24 inch advancers and is going to be fired in am to cone 11- 1/2 way down gas reduction fire in AM.

I place the pots so they almost touch and have forever with no sticking to one another. Refires will expand more and need more space as you can see they are a few in this load. As far as top space as close as I can get a shelve to clear with my nerds is what works for me all these years.

For those of us who make a living at this more in each kiln load is a key point. This may not work for everyone but it does for me which is what counts. My glazes do not expand more than almost touching the next item/shelve.

Every few years I will stick a pot to another-usually because I have shoved them together after loading them by moving them after placing them the 1st time around around. When you load a kiln like this many times all this is second nature-how tall of a stilt is needed what is the hieght of wares etc.This kiln loads in about 1 to two hours depending on how much small stuff is in it-I'm not a numbers guy counting pots but there is over 400 pieces in this load I'm sure.

Mark

Thanks for this, Mark. A picture is worth a thousand words.

T.

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Pugaboo    438

Oldlady - "don't be afraid, newbies, it will some day become as familiar as your toaster."

 

Lol I just realized my toaster has a Cancel button last week so rather than yanking the lever up on the side to get toast out as its just about to burn I now push the little blue button that's says cancel. I have owned this toaster for a year and a half; lets hope it takes less time to figure out the basics of my kiln!

 

It only took me 4 times this time around to fit everything in to the kiln for my 2nd bisque load. My husband is now thoroughly convinced I am crazy... Well crazier than he thought... Since I leaped out of bed around midnight last night to race down and reload the kiln to get a better balance. That tall vase gave me fits and I have come to the realization that my plan of 2,4,6,8,10 and 12 inch high posts was a WRONG buying plan. What I really need are a bucket load of 5 inch posts and a dozen 1 inch posts for when all else fails and I am trying to get a shelf to spiral around the ones below it and none of my posts are the right length. I did figure out that by laying them on their sides and standing another post on top it all works out fine hence my dash out of bed at midnight to go mumble to myself in the garage as I unloaded and reloaded the kiln for the fourth time. I don't think the pots minded my pajamas at least they kept quiet about my bizarre attire until I left the room after all as oldlady said nobody REALLY knows what goes on in there once we shut the lid and leave the room.

 

The load fired through to completion and is now cooling off I'll be able to open it tomorrow afternoon and see what's what. Then gee guess what we get to do my first GLAZE FIRING! Ooo what fun... No you are NOT allowed to sign out and avoid the forum for a few days just to avoid my annoying newbie questions! I'll be good and try and keep them to a minimum... Remember now I said TRY LOL.

 

good night all and happy firing!

 

Terry

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Mark C.    1,808

Terry

You can bisque pots on thier sides as well with no issues. Pile them up or stack them high.They are tougher than you may think.

Mark

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Mart    23

 How does heat-work tally up? If you fire and empty kiln to a temperature (or cone), and fire it again to the same endpoint full of wares, how much percentage wise more power is used? I think I'm having calibration issues with my sitter or I'd test instead of ask, but it seems to me, over my three firings so far, that the one full load took 400% longer to fire. I'm really unsure about the sitter though...

 

(Sorry if this is off-topic, but several posters mentioned filling the kiln to save energy.)

 

If I recall, it's 360 Btu/h per 1 lbs or ~800 Btu/h (0.234 kWh) per 1 Kg ... but do not take my word for it :)

Fact is, heating up 100 or 150 units of something, takes different amounts of energy... unless physics is different in your galaxy :)

Do not forget, that kiln(interior) needs to be heated up too. If your load is only a fraction of kilns weight (includes shelves, bricks, posts etc) then it will not really matter much at all.

I have a small kiln so I can actually notice the difference. Question is, what is the optimal kiln size litres / load in litres (<max weight) ratio.

I try to keep about 1/4" or 3/8 gaps. Biggest waste of space comes form narrow 15 cm piece surrounded by 10 cm items on the same shelf and having the next shelve staring at 16 cm.

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neilestrick    1,381

A digital kiln will fire in about the same amount of time whether it's full or empty, because the controller is keeping it on a specific ramping schedule. It will use more power to keep it on schedule with a fuller load, but the overall time will be about the same. On a sitter kiln the firing time can change dramatically depending on how full the kiln is. But to save energy, money and labor, you should try to fit as much as possible into a load no matter which type of kiln you have.

 

Gas kiln can be packed very tight with no problems, assuming it's designed properly.

 

Wood burning kilns are an entirely different beast. Too tight and you can choke off the air flow. Plus you need space for the flames and ash to move around.

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My point was just concerning the energy efficiency. Since you wouldn't fill a kiln with furniture if it had only a small load of wares, and nothing else in there absorbs much heat, your energy efficiency is not greatly affected by the density of fill. Possibly, it may even work the other way, that a large load would be less efficient because of the furniture, and because of the insulating effect delaying the radiant heat propagation.

 

But I'm a rank amateur, probably I'm just not getting something...

 

As for production potting, sure you probably want to turn out as many pots as possible, and the energy question is only one part of the picture.

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