Base isolation of wood fired kiln

[Alyosha]

Hello,

According to quite a few kiln builts, hollow concrete is used as foundations to chase away the humidity and then refractory bricks are layered on top. Would you add any over isolation ? I was thinking of two options : either adding vermaculite with china clay in the hollows of the concrète blocks, or addind a layer of the same mixture on the surface above, on which the firebricks will be added. What are your thoughts ?

[neilestrick]

Cinder blocks are used as the foundation of kilns because it allows for air flow through the blocks. If you build directly on top of a concrete slab, the heat from the kiln may calcine, spall, or even blow up the slab. Do not fill the openings of the cinder block, and there's no need to lay anything else between the cinder block and firebrick. Lay the cinder block so the openings are horizontal, not vertical, so air can move through.

[Alyosha]

Thanks again Neiles,

I have thought of using a bed size bloc of polystyrène  with large slabs/sheets of concrete on top, both I have at disposal. After thought isn't there too much risk the heat might propagate through the firebricks, to the concrete and then to the polystyrène which would be catastrophique ?

Can large hollow orange bricks be used instead of the cinder?

 

[Mark C.]

The kiln floor with have more heat than most imagine.

I have hollow cinder bricks under my cone 11 car kiln- it lets air flow under. even with that and about a  8 inches  of clearance under floor the slab of concrete still gets warm.

Three floor layers of bricks on top of that. floor is 7.5 inches thick

Wood kilms fire for longer periods and time is what allows heat to travel slowly thru

My fire averages 12 hours up and 48 hours to cool-60 hours and the slab is still warm.

Polystyrine is not a refractory material and has no place in kiln construction-only use materials that are durable and can take heat and weight and not melt or burn or compress.

Red bricks also are not for kilns except mabe on a 3 third layer for astethics. Do not use them under floors.

A far better materail is super heavy expanded metal mesh -I have that on top of my cinder blocks turned on the 16 inch side (meaning cinder blocks are standing on ends a few inchs apart ) thein a layer of 3/4 inch heavy expanded mesh as a floor layer -then one layer of hard brick then a layer of soft insulating brick then a layer of hard brick-they are laying on the 2.5 inch sides so thats 7 inches of floor layer over the expanded metal. The floor is air cooled with this setup and kiln is up a bit sot its easyer to load. Sprung arch flat top. This is for my gas fired high fire salt kiln. I was experimenting with some new ideas and most have worked well.

 

[Alyosha]

Hello @Mark C.,

Thank you for the generous reply.

In the design of the kiln I am building there is a layer of perforated red bricks as floor for the kiln, the bricks laying on there side underneath are also perforated red bricks. They could be replaced  by super heavy extended mesh instead if I've understood correctly ?

In this design when stoked an extremity of the wood falls halfway on either side on two horizontale steel tubes so they burn almost verticaly.  Diagram  of the sideview of the firebox >       [ '.\../.' ]

 

 

[Mark C.]

I'm confused by that photo-does the ash fall thru the bricks with holes onto the cement cinder blocks? If so those  than  the cement cinder blocks are pretty close to a huge heat source-not to sure about them holding up to that heat.

In my design the expanded  steel sits on top of the cement cinder blocks-all fire bricks sit on top of that.

There then are 3 layers of bricks that make up the floor or 3 layers of bricks away from the steel

its not a wood kiln. Its a Salt kiln

[neilestrick]

You really don't want direct heat on the cinder blocks. Cement and high heat don't mix. I'm confused about this design as well. Will ash and embers fall through the perforated bricks?

[fergusonjeff]

I used hardy-backer (like you would use under tile) over the concrete blocks to create  more level surface.  then used a layer of softbrick under hardbrick floor for extra insulation.  The perimeter (under the walls) is hardbrick. 

[Alyosha]

Hello @fergusonjeff,

Ok, I have large concrete sheets I could use in order to level the surface.Thanks. Ivdont understand exactly how the perimeter  differs from the sole base ?

@Mark C., @neilestrick,  after reading your arguments I am intrigued. Here is a link to the design, the publishers have a free e-edition as well as a booklet that can be bought:https://fr.scribd.com/doc/271763553/backyard-kilns-steve-mills-1 

I'll check if there's any mention of the subject,

Thanks 

[neilestrick]

1 hour ago, Alyosha said:

Here is a link to the design, the publishers have a free e-edition as well as a booklet that can be bought:https://fr.scribd.com/doc/271763553/backyard-kilns-steve-mills-1 

That site requires a signup with credit card to read it, which I'm not going to do. Let us know if you find any specific information on how this kiln works. We're not saying it's not going to work, it just that this  isn't the typical tried-and-true method for building a wood kiln. Personally, I'm skeptical of designs that use less-than-ideal materials because they don't last as long, there can be safety issues, and it generally means that the designer used those materials because of cost issues, which is not a good way to design a kiln meant to go to stoneware temps. If this was a low-fire raku type kiln then I wouldn't worry.

People often underestimate the requirements for kiln materials. Most bricks cannot handle the extreme heat of a ceramics kiln. That's why we question the use of common bricks- they're just not made to go to 1350C. And if they fail, there is the risk of structural failure and injury. Also, there's a lot of information on the internet on how to do things (like build kilns) that work for the person who put the info out there, but isn't necessarily a safe way to do it. Plus if you're not sourcing the exact same bricks from the same manufacturer, your bricks may not hold up as well as his bricks. Small differences in manufacturing can have huge results in how they respond to extreme heat.

So let us know what you find out about the kiln. Worst case, you could use the kiln for lower temp firings instead of cone 10.

[Mark C.]

Here is better link to kiln book without joining anything

https://stevemillsmudslinger.weebly.com/backyard-kiln-e-book.html

After review the photo you have included is a bit different from the layout layer by layer on page 14 which shows a solid floor.And its two layers I think

Its confuzing at best-I have no argument with this kiln except if hot ashes fall onto the cement blocks thats not going to work well-concret does not play well with heat.

These perforated red bricks appear to be regular red brick which is not going to take heat well either. Some in book appear brown like fire bricks some appear red ?

I'm more confuzed now than before

What I do get is the kiln is made for a  limited life 

Let us know how it works and post some photos on the build

I have built a wood kiln with shelves for the roof  a bit like this one once in the 70s for a single fire. It did not make two fires . Of course we where having fun learning and did not have enough of the right materials to build a proper kiln. Life as a 30 year old potter learning what works.  We melted the steel firing crates Now its 12-15 kilns later and I know what does work

 

I do have a story about a kiln build on a fresh  concrete slab with not enough hard bricks for a floor. I was there whan a potter friend in 1975 buildt a catanary arch kiln with one or teo layers of bricks ontop of that feshly poured cement slab. -Well the slab cracked with what I work call an explosion -runied the kiln and most of the wares.

[neilestrick]

I've heard several stories of cement pads blowing up from the heat of the kiln, too. It happened at my grad school a couple years before I was there and you could see the patch in the slab.

I've built kilns with kiln shelf tops, and they work, but they have a limited lifespan so it'll get expensive if you plan to use the kiln for a long time. Flip the shelves each time and that will help keep them flat.

[Alyosha]

Thank you very much @Mark C. For pointing out the better link to the booklet. It is also on google free books : 

https://books.google.fr/books/about/Backyard_Kilns.html?id=xcVtCAAAQBAJ&printsec=frontcover&source=kp_read_button&hl=en&newbks=1&newbks_redir=1&redir_esc=y

I have contacted the kiln builder who advised me to build this kiln to lighten up the matter.  He also has a kiln build that looks very good but there are no plans out available as clearly instructed as the backyard kiln, which is convinient for me as a beginner and also as I have to make with only 650 bricks,

Hi @neilestrick, Regarding the backyard kiln, I've looked for other solution for the top of the kiln like making slabs out of vermaculite and china clay or was thinking of making an arch , or moveable flat top with  the extra bricks instead of the chimney (the kiln requires 380 bricks with a metal chimney -which I've read is less surely efficiently than a brick chimney ) but I haven't found enough information about the alternatives. Thanks for pointing out about there fragility and way to handle them.

Here is the design of the potter who told me about the backyard kiln, it seems to be made of more bricks. It is published on the website of another potter here : https://elinhughesceramics.wordpress.com/2018/07/23/kiln-building-with-joe-finch/

Best Wishes,

[Alyosha]

@neilestrick Thanks for warning me about the risks. Apart from the concrete slabs and eventually the perforated bricks( which I will not use now warned if I do not understand how it could in fact work well) the materials are proper firebricks :

1/3 IBS and 2/3 dense but no so heavy beige refractory bricks 

as well as 50 heavier beige dense refractories with more of an orange hue to them.

I also have 20 large and very heavy dark red refractories from a heater. If I remember well you advised me to only use them the outside (or perhaps on the concrete slabs as extra layer then?) as they are surely high in iron.

Although I have no spécifications about the bricks I have I believe by their aspect that they are more refractory than those in the photos of the booklet.  Guess I'll soon find out.

 

[neilestrick]

After looking through the plans, there are several things that I do not like about the design. In addition to exposing the concrete blocks to heat, the bricks he used in his design were not made to handle the temps he was firing them to. He specifically says that their shrinkage after firing is 'considerable'. That's a sign that they shouldn't be fired that hot. Plus if your bricks shrink a lot, then the kiln will no longer be tight, and an arch would no longer be stable. It also appears that he built the kiln with walls that are only 4.5" thick. Walls that thin leak a lot and don't insulate well, and are likely terribly hot to the touch and possibly a fire hazard, and are not very stable. He also recommends using fencing wire to hold the kiln together. Wire is not strong enough to hold a kiln together. There is a fair amount of expansion that takes place when the kiln is hot, and wire will either stretch or break. If it breaks when someone is near it, it could injure or even kill them- imagine a piece of wire whipping out at high speed. Very strong cable would work, or angle iron or threaded rod. Wire of any sort is not sufficiently strong.

The over-all concept is good in terms of a kiln that will get to temperature, but the execution is not great. Thicker walls, proper bricks, and proper reinforcing are needed in order for this kiln to be durable and safe.

[Alyosha]

Thanks you Neilestrick for the feedback and especially for the safety measures, I will have to double up the wall with the 270 bricks lefts instead of making the chimney out of bricks and soulder the irons angles with irons flat bars on sides in addition with the turnbuckle  110kg wire system. I am still perplexe about the concrete flooring I have called a couple of potters in France who will also give me their feedback. I could use the red iron refractory bricks found in a heater on top of the concrete slabs and go for the non perforated version of the kiln with on or two layers of dense refractory/ibs bricks.

Here are photos of the bricks a mentionned in my previous post and  a large amount of tiles,

[neilestrick]

I can't really say based on the look of the bricks. Ultimately you'll have to figure out which brand they are and if they can handle the temps of a wood kiln. In general, concrete should not be in direct contact with flame/fire, and any bricks in direct contact with flame need to be rated for the high temps of the kiln. The firebox is generally the hottest part of the kiln.

[Alyosha]

Ok, so the design of the perforated brick kiln is quite particular then.

[Mark C.]

I agree with Neils assessment-not a safe kiln but thrown together with low cost materials not really made for the purpose.

The safety factor is an issue.

ON the bricks the more red means the more iron and that means the less heat they can withstand . Fire bricks are tan not red.

Any kiln with hardbricks 4.5 inch thick is really not a cone 10 kiln

These facts are true worldwide

[Alyosha]

Ok  thanks Marc and Neiles.I’m surprised about all this because the potter who recommended this design for me as a beginner seems to have worked alot on building and firing kilns and has published a book on them. I’ll be carefull and patient before constructing this kiln, gathering more information and advices and if I do build it I’ll double up the walls or put a layer of fiber blanket aluminium foil and metal grid. Can the second row exterior be of bricks on their side or that thickness will not be enough ?

[Mark C.]

Cone 10 kilns are usually 9 inch thick walls-you can do less but you need a layer of insulating brick  in that mix to keep the heat in.

That means a kiln floor for example could have a 2.5 inch layer of hardbrick then a 2.5 inch layer of insulating soft brick then a layer of 2.5 inch hard brick-total floor thinkness is 7.5 inches thick -this has worked before for me. That was gas kilns

 

wood kilns due to the wood buring on the floor and the hot ashes really heat up a floor as well as the firing times tend to go longer (more heat for longer on floor) Wood kiln are to tough on soft brick so if you use any its needs to be not on the hot face

I think another book on kiln making would do you some good as well so you have more of an idea on all of this. Its best to read many books on kiln making before building a kiln to see what has worked and whats best practice .

I have fired at a distant workshop once a kiln that was 5 inch wall of soft brick (insulating bricks on edge) to cone 10 and the wall was hotter than heck-it was a homemade kiln donated to an art center. It had a short life as it was poorly designed with walls to thin .The metal on the outside gave way to heat and rust.  I converted it  into a salt kiln and after a few fires that killed it.

You can make a kiln many ways but there are some rules that make them work better and last longer.

[Alyosha]

Thank you Mark C,

I understand the kiln in the booklet is not made to have a long life perhaps it's more of an event kiln. 

About the kiln floor I am thinking of perforating concrete sheets and putting on top of the cinderblock. Either adding the dark red heavy low refractory iron bricks on top plus Ibs and then the dense refractory firebricks or only using the last two. 

I will post photos here as soon to show the setup

Well regards,