2017 MHA Annual Meeting

36" Castable Refractory Oven Experiment

with Marty Pearson and Norbert Senf

People
36" Castable Refractory Oven Experiment
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Squirrel Tail Oven
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Batch Rocket Cookstove and Heater
HMED 22" Contraflow
Small Finnish Contraflow for Beginner Masons
Quick Grill by Chris Prior
Tulikivi TU-2200 Top Vent
Ecostove
Tiileri Brick Heater Kit
Archguard demo
Presentations

Pile of damp sand. Sand that packs well is the best. This sand was OK, but not great.

Compacting the sand.

Starting to form it into a 36" hemisphere shape.

Making sure it is damp and there is no dry sand, which does not pack.

Establishing the baseline

Levelling the baseline

Forming the front opening.

An impromptu cardboard template was very handy for getting the shape.

Forming the front edge of the oven opening.

Forming a dam at the bottom to delineate the 2" castable concrete thickness. We calculated that it would take 8 bags of castable refractory (50 lbs each).

Damp sand form is ready.

Sand casting is a technique that was widely used in the 1930's to cast full dimension artificial stone. The basic idea is that you use a very wet concrete mix, and that the sand will suck out the extra water, giving you a normal strong concrete (extra water weakens concrete, and in particular castable refractory, which is high heat concrete).

Normally, you would make a mold into a mass of damp sand. In this case, we have reversed the procedure and are plastering onto a form. We discussed this on the MHA members Yahoo List, and wanted to see how it would work.

We purchased the castable refractory at the Asheville pottery supply for $40.00 per 50 lb bag.
It was "Econocast" manufactured by Pryor Giggey Co. Pretty much any general purpose castable should work the same.
I made the first batch fairly runny, using the "ball in hand test"
It looked like there might be an issue with the concrete adhering to the mold, so I made the second batch stiffer, more like normal "ball in hand" consistency. Note that the first part is almost vertical, and the hardest to do.

Further up, we made a slightly wetter mix.

Pulling the form for the front of the opening. The sand was sucking out the water nicely, and the concrete was stiff enough to trowel.

A 7" chimney opening was formed using a piece of pipe.

Finished.
You want to resist the temptation to over-trowel it to a smooth finish, as this retards the drying process. A wood float finish is recommended.

We cut the oven shell into 4 pieces, using an old hand saw.

It was left in the open air to dry until the first patches of surface drying were apparent.
Then, it was covered with plastic overnight
Castable refractory uses calcium aluminate cement (fondue cement) as a binder. This is a super high early cement, and is fully cured in 24 hours..

The next morning.

Notice the honeycoming. This is from too stiff a mix.
In discussion, the consensus was that there really was no issue with adhering the wet mix, and therefore the mix could have easily been wetter.

Oven base with 2" calcium silicate board insulation.

The pieces were mortared together with clay-sand mortar.

Cleaning up the flash on the door edge.

Lighting a curing fire on the morning of the pizza party (Friday)

Fire was kept on low all day, and then cranked up around 4:00 for the pizza party.

Success!
We fired the oven very hard, to see if we could break it. We could not. There were some small hairline cracks in the clay mortar joints, which is exactly where you want them.

See also:

2007 Photo Report
2006 Photo Report
2004 Photo Report
2003 Photo Report
2002 Photo Report
2001 Photo Report
2000 Photo Report
1999 Photo Report
1998 Photo Report
1997 Photo Report

This page was last updated on August 12, 2020
This page was created on April 18, 2017

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