Should I mix anything with my clay wall layer?

[mattyana2003]

I am building a clay oven in my backyard and would appreciate some help. I have built the foundation and sand form and am ready to put the clay layer over the form which will be 6" thick with a cow dung/clay/sand stucco over the outside which will be 1" thick. I have bought Buff Raku clay from a pottery supply which has a high grog (ground fire brick) content. I am not sure weather to mix any sand with this and if so in what ratio. I have heard varying reports as to weather this is necessary or not because of my high grog content clay and that the sand will keep falling into the food as the oven ages. I intend to wet the clay down to a more pliable consistency before use and would prefer not to add sand as this is more work. Will pure groggy clay crack easier than the same clay mixed with sand?
Please help,
Mattyana

[Archena]

Re: Should I mix anything with my clay wall layer?

I'm not sure - I've always thought sand was the filler and that straw was the binding agent when used. I don't see why sand should increase binding.

[dmun]

Re: Should I mix anything with my clay wall layer?

Grog, at least when I was a potter, was fired clay, ground in a ball mill then sifted to the proper size granule. I don't think it has any specific alumina content like refractory products, unless it's something special. It's intended to add strength to the fired clay body, not provide any temperature resistance.

Manure? In an oven? I know this is traditional, but I hope you are going to cure this with long, hot fires before you put any food in the oven. Won't it, you know, smell?

[Archena]

Re: Should I mix anything with my clay wall layer?

No, manure only smells when fresh - it's urine that won't lose its smell. Dried manure has little or no odor and doesn't regain it when rehydrated.

Don't ask how I know that...

[RTflorida]

Re: Should I mix anything with my clay wall layer?

Its amazing what you retain from watching the Discovery Channel
We certainly don't think it is from firsthand experience

[dusty]

Re: Should I mix anything with my clay wall layer?

I don't even WANT to know!

dusty

[asudavew]

Re: Should I mix anything with my clay wall layer?

No, manure only smells when fresh - it's urine that won't lose its smell. Dried manure has little or no odor and doesn't regain it when rehydrated.

Don't ask how I know that...

So how do you know?

Someone had to ask..

[rcspott]

Re: Should I mix anything with my clay wall layer?

A little info

Handy Home Projects: final plaster layer for the mud oven - clay manure mix

Rich

[Guerito]

Re: Should I mix anything with my clay wall layer?

asudavew,

My wife knows. Born in Mexico; her family was poor, and everyone worked on the ranch. When she was little, they would burn dried cow pies as fuel for their oven. Each family that worked on the ranch had their specific section where they could gather the dried cow pies. No odor!

Guertio

[Carl]

Re: Should I mix anything with my clay wall layer?

I'm going to use a recipe from John Conrad's book "Contemporary Ceramic Formulas" to make my oven walls. The book contains a whole chapter on refractories, and has a range of recipes to make refractory materials, including firebricks and crucibles.

The recipe I'm using will be 70% firebrick grog, 25% fireclay, 5% ballclay. The firebrick grog is just ground up firebricks, so this stuff is as close as I can get to 'mould your own firebrick'. The alumina content of the grog is high, so it should hold heat just like firebrick. I'll report back here once I've done it - I'm working on a roof for my oven right now so all the clay doesn't wash away in the rain...

As I understand it from Kiko's book, the thermal expansion rates of sand and clay differ, so they can lead to sand separating from the clay when heating and cooling. That's why I'm using grog, rather than sand. Sounds like your clay is well grogged - can you find out what percentage grog it is?

Here's a link to someone who's done similar with a grog, clay and fireclay mix: Backyard Boulangerie I'm copying parts of this design, while trying to add a few improvements too.

Carl

[james]

Re: Should I mix anything with my clay wall layer?

Wattle and lathe.

I'm out of my depth here, but the walls of Tudor houses were framed with wood lathe (I am guessing oak strips) and the "mud" was manure, straw, and clay -- which was finished with lime wash. Or something like that. I've had too many pints in 400-odd year old pubs, and the walls still look as good as new. At least they do after a few pints.

Can our British contingent comment on this one?

Then, perhaps we can talk about maintaining a thatched roof. :-)
James

[dmun]

Re: Should I mix anything with my clay wall layer?

Did you know that "lathe" and "Lath" come from the same root word, in middle English, that means "twig". It comes down to us as the wood strips (or expanded metal sheet) that are secured to a structure and hold plaster or stucco.

In the old days, a green tree, or lathe, would be tied to a rope and bent down to provide a return spring for a foot powered wood lathe. The rope would be wrapped around the workpiece between centers, and the turner would cut on the down stroke.

Here's a modern take on the idea, using a bungee cord instead of a green tree.

Thus ends your trivia lesson for today.

[CanuckJim]

Re: Should I mix anything with my clay wall layer?

Dmun,

Nope, extended trivia lesson. Before dedicated lath mills started operation, here in the late 19th century, to produce identical pieces of oak for plastering, irregular slabs of, unfortunately, green white oak were partially split with a hatchet, then pulled apart a bit to form key areas for the plaster. It's called "accordion lath," and is one of the ways historians can date houses. Then there are the fasteners: square cut nails or blacksmith forged nails. How dat for trivia?

Jim

[Archena]

Re: Should I mix anything with my clay wall layer?

Did you know that "lathe" and "Lath" come from the same root word, in middle English, that means "twig". It comes down to us as the wood strips (or expanded metal sheet) that are secured to a structure and hold plaster or stucco.

In the old days, a green tree, or lathe, would be tied to a rope and bent down to provide a return spring for a foot powered wood lathe. The rope would be wrapped around the workpiece between centers, and the turner would cut on the down stroke.

Here's a modern take on the idea, using a bungee cord instead of a green tree.

Thus ends your trivia lesson for today.

Psst! The link doesn't work - unless you were intending to include Arctic research in your trivia....

[dmun]

Re: Should I mix anything with my clay wall layer?

Psst! The link doesn't work - unless you were intending to include Arctic research in your trivia....

That's pretty strange, I just pulled that page up from google images. Here are the images, anyway:





and a bit of the text:

The first photo shows the entire lathe. The main body of the lathe consists of a pair of 2 x 6 beams that form the bed of the lathe that are bolted on either side of the 2 x 4 legs that raise the bed to the appropriate height. Carriage bolts hold the bed beams and the legs together, and keep the unit square. A base of 2 x 4's supported by diagonals keep the lathe supported perpendicular to the main frame.

The basic dimensions depend on how tall you are, and the length of the longest thing you're going to want to turn. My lathe is 40 inches wide, which allows me to turn spindles up to 26 inches long. The top of the bed is 42 inches off the ground, the poppets are eight inches tall, with seven inches between the top of the bed and the point of the dead centers in each poppet.

The poppets are made from three pieces of 2 x 4 glued and screwed together. The middle piece is long enough to extend down through the bed and below, and has a large tapered mortise in it. A wedge is fitted into this mortise, and when the wedge is driven into the mortise, it pulls the poppet down and holds it in place in the lathe bed.

The dead centers themselves are mild steel round bar, 3/4 of an inch in diameter. I turned the ends to points with a file, using my treadle lathe to spin the rod. Roy Underhill showed a similar operation using a post drill, and I imagine a drill press would also work. You don't want a long taper because it will drive itself too far into the workpiece. Instead, you want the taper to be about 45 degrees, so the distance from the start of the point to the point itself is about the same as the radius of the rod. You can see how blunt the center looks on my lathe, even though the point itself is pretty sharp.

The spring pole part is actually a bungee cord that is stretched parallel to and above the lathe bed. The rope that wraps around the workpiece is loosely tied to the bungee cord. You can control how much effort it takes to push down on the pedal, and how quickly the lathe returns by how much you've stretched the bungee. The ability to move the rope across the bungee is useful so you can place the rope on the workpiece away from where you are presently turning.


The pedal is just a long flat board with a piece of cloth stapled to the underside. The cloth is then stapled to another piece of wood that you'll stand on with your inactive foot. You could put a metal hinge here, but the cloth allows you to adjust where you're pulling the rope and also helps keep the rope away from where you are turning.

On my lathe, I never bothered to make a true tool rest, but simply clamp a piece of wood to the poppets, with the top edge of the board set at about the height of the dead centers. It's not as useful as a true tool rest that could be moved closer to the workpiece, but it does work. A traditional tool rest would look sort of like a third poppet, but much shorter, and with a movable extension on it that can be moved toward and away from the workpiece, and has a rest that is the same height as the dead centers.

The first picture shows the lathe all set up with a piece of birch between the centers. And the last picture shows a close up of the turning setup. The rope goes around the workpiece twice and is then tied to the bungee cord such that the pedal is in an appropriate "up" position. The rope wraps around the workpiece in such a way that when you press down on the pedal, the workpiece spins toward you. This is the cutting stroke, and is when you're moving the tool ever so slightly into the workpiece. When you release the pedal, you pull the tool back slightly so it's not riding on the wood when the bungee pulls the rope back up. The slight movement back and forth quickly becomes habit, but it's difficult to remember at first, especially while concentrating on how you're actually using the tool.

To get the workpiece ready for turning, I scribe a circle on the ends with a compass, and chop down close to these lines with a hatchet. A draw knife brings the piece closer to a cylinder. Before putting the piece between the centers I like to drill a very shallow hole at the center point for the centers to ride in. If the piece works it's way off the centers, you may need to make this hole deeper and wider so the piece stays on the centers.

You'll also notice in the last photo that the rope is wrapped around a narrower section in the workpiece. It's often helpful when turning a large diameter cylinder to start by turning a narrow groove for the rope to work in. Not only will it keep the rope from moving into your tools, but the smaller the diameter where the rope is, the faster the workpiece will turn. A large diameter cylinder with the rope wrapped around a very narrow section can be spinning really quickly, and this often makes turning easier.

Finally, keep in mind that turning with a spring pole lathe is much easier with wet wood. This style of lathe was typically put together in the forest by spindle turners so they could cut down, chop, round and turn chair parts right on site. The first time you try turning wet wood instead of dry, you'll understand why they did it this way. You certainly can turn dry wood on a spring pole lathe, but don't expect it to be easy.

Photo and text credits: Christopher S. Swingley