Oven from large pottery urn

[olNick]

Hello to all,

I'm Nick and my first post here.
Friend has acquired a large urn, over a meter tall, and wants to make an oven and asked for my help.
Would prefer not to risk shattering the urn by cutting it in half lengthwise, so I would like to ask if you foresee a problem with setting half the urn into a raised concrete base, and then insulating the inside of the bottom half and laying the firebrick floor in there.
Of course the bottom of the urn would also be supported with a cinder block under it

Like the pic attached (concept only :-)

The exposed top half would be insulated and rendered as usual.

My main concern is that since the top half the urn is getting fired and bottom half is not, the urn might crack, shatter? Possibly where it joins the slab.

Urn is something like the other pic I found on the web...

http://www.digbyhoets.com/images/Por...large_urns.jpg

Regards,

[mrchipster]

Re: Oven from large pottery urn

A few questions. How do you plan on cutting the door if you are concerned about cracking the urn when cutting it?

How thick is the urn and what are it's basic dimensions? You mentioned height but not diameter. If it is thin which it probably is you will want to add thermal mass

I would agree that the temperature difference between top and bottom may cause cracking

I had a ceramic/clay chimiinea and it cracked badly after a couple of years. And it was never subjected to the kinds of heat a pizza oven gets to.

Have you considered a cob oven? If you are concerned about cost this may be a better way to go.

[oblertone]

Re: Oven from large pottery urn

It'd be far better suited to making a tandoor oven; have a look on you-tube for clay pot tandoor and you'll see what I mean, it's still cooking with fire !

[scott123]

Re: Oven from large pottery urn

As I sit here typing this, I have a chipped tooth that was caused by biting into a slice of pizza that had a small shard from the top of the oven fall into it during baking. Because of this, I tend to be warier of makeshift oven materials than most.

That being said, this particular urn is especially sketchy, imo. I'm no fan of regular clay non fireclay bricks, but, generally speaking, most clay bricks are at least reasonably well sintered/fused. The larger the item, the more difficult it is to fire evenly. An unevenly fired urn may be perfectly fine for decorative uses/planting uses, but once you start exposing it to heat, these internal variations in sintering are going to reveal themselves with cracking quite soon.

The manner in which the urn is crafted will make a difference as well. Clay bricks are generally formed using some pressure, which produces a denser, stronger end product. When you get into other forms of ceramics, you're looking at a few different forming techniques, each providing varying levels of strength. Hand built items tend to be physically strong, but, for an item like this, you're looking at a coil technique, which, imo, will likely produce thermally weak areas where the coils meet (variations in density will be just as bad as variations in sintering that I mentioned above).

You might find an urn like this being thrown on a wheel, but it most likely won't be from a single piece of clay (better from a thermal perspective), but, rather, from starting off with coils. If it were a single piece of clay, it would involve world class throwing skills, imo, and be prohibitively expensive. The last possible way this could be formed is by casting, which produces the weakest/most porous end result of them all.

Not that I'm even certain this urn is fired clay. It looks like clay, but if I'm wrong and it's just cast cement, then you absolutely do not want to use it for an oven, since cast cement is the thermally weakest material of all.

So, long story short, some people (not me) are okay with regular clay bricks being used in ovens. Regular brick vs firebrick argument, aside, though, this urn is a very long way from a clay brick in terms of potential durability in a high heat setting. Don't use it.

And this same advice applies to tandoors as well. While you can find quite a few DIY tandoors online that have clay liners or are made with refractory cement, the common denominator for just about all of them is cracking/spalling. Because of the vertical nature of the oven, spalling isn't going to drop bits of the oven onto the food, but you can just as easily see a piece flake off and get lodged into the bottom of the bread. A proper tandoor, just like a proper wfo, should be firebrick.

[Tscarborough]

Re: Oven from large pottery urn

Why do so many people work so hard to avoid the obvious and not just build the damn oven as it has been refined down through the millennia?

Sure you can use a flowerpot and a crackpot scheme to cook something, but why? You can spend a little time and some money, or a little money and some time and build something that not only works, but works well and will prove to be an object of both pride and utility, as well as an asset to the real estate instead of something you use once or thrice and then don't use because it doesn't work well and have to tear out before you sell.

It makes no sense.

[olNick]

Re: Oven from large pottery urn

Thanx to all who have responded,

WRT the door in the first reply, there is no need to cut anything as the urn is placed on its side and the (half) opening at the top is used as a door/entrance to the dome.

WRT insulating etc. I stated that it would be insulated and rendered as usual...

As I said, this is not my project, only helping a friend who has his mind set on this and will not build a WFO as is usually done. I'm just trying to advise him a bit and improve his chances.

As an aside to all of this, using an old urn as an oven used to be quite common here on Crete (150 years or so), and that is where his idea originated. I have seen a few of these and some are still working but I did not check whether they are cracked.
Most of these installations though are built quite low, around a foot, foot and half off the ground and the lower half of the urn is buried in soil.

This is probably why all the old grandma's that still use these ovens are all hunchbacks :-)

Most of these urn were used for storing olive oil in the old days and some I have seen are glazed.

Thanks for the informative breakdown of the differing pottery Scott.

I did not mean to agitate you Tscarborough and I'm aware tof how it's been done for millenia, that's how I got to this forum in the first place and at least I'm not asking on reusing an old oil drum...

I will relay your responses to my friend and i will also look a bit closer at this urn.

tx all for your time,

nick

[Greenman]

Re: Oven from large pottery urn

Like Tzarborough said and do it once and do it as well as you can. It doesn't need to be expensive if you are prepared to do the yards in sourcing the materials and do the work yourself.

It really is worth the doing and even the next owners will love you for the work put into building a decent oven.

[david s]

Re: Oven from large pottery urn

As stated by Mr Chipster the temp difference between top and bottom is likely to cause cracking. I would recommend cutting the thing in half. It is quite easy to do if you use a 5" angle grinder with a diamond blade. (A 4" grinder may not give enough depth of cut).

Done carefully, you may end up with two that you can use.