What sand?

[lorettaf]

1. I’m going to build a sand/ powdered fireclay dome. I have available a dumpy bag of what i think is sharp sand from jewsons. It’s grey and is pretty coarse - Is that the right stuff or will it spit grit? Or do i forego my freebie and buy the yellow finer soft sand you see around for play sandpits? Does it matter? I could use the coarse sand for the mold and the rougher cob outer layer.

2. Also I’ve got Eric Wisner’s earth oven book and he suggests 3-5/1 sand/clay slip. The advice on this forum is 2/1 by volume which is a huge difference - any advice? Is it the difference between using dry materials vs dug up clay? I was going to make bricks and dry them. Is 2% shrinkage the max - or too much? Thanks

[fox]

The best budget mix for a wood oven dome is based around what is termed as Home Brew

The most proven formula for homebrew is 3:1:1:1 (By Volume) of Sharp Sand, Hydrated Lime, Portland Cement, Fire clay respectively.

You can go one step further with the addition of SS needles (2% of dry material weight) for reinforcement and polypropylene Fibers (about 1%)
Easy to use and proven to last many yeats.
.

[lorettaf]

Hi Thanks Fox, i know lots of people here like the homebrew mix but i was going to go a bit more old school....
I’ll see how i get on. But thanks for replying.

[downunderdave]

Aug 20, 2021 15:05:22 GMT fox said:

The best budget mix for a wood oven dome is based around what is termed as Home Brew

The most proven formula for homebrew is 3:1:1:1 (By Volume) of Sharp Sand, Hydrated Lime, Portland Cement, Fire clay respectively.

You can go one step further with the addition of SS needles (2% of dry material weight) for reinforcement and polypropylene Fibers (about 1%)
Easy to use and proven to last many yeats.
.

Hi Fox, I think you've made an error with the fibre addition recommendation.

Stainless needle (melt extract fibres) reinforcing is recommended to be between 2-4% addition by weight. 2% is on the light side.
The polypropylene fibres are extremely light and are not used for increasing strength as they burn out. addition is more like 0.1% by weight (actually on the heavy side). In practice around a small handful for every 10L dry material is adequate. However a longer mixing time is essential as the fibres tend to clump and require extra encouragement to disperse properly.
Also, as the stainless needles  are difficult to work with, (they're not called needles for nothing), AR (alkaline resistant) fibreglass fibres as used by the concrete countertop guys, can be used as an alternative, or in addition to the stainless needles, as their melting point of around 900C is way higher than the oven's service temperature range. They are far easier and kinder to work with.

[fox]

Ah yes sorry about the fibres … 0.1% is what I meant to say, the comercial refractory mix that I use specifies 2% ss needles in their mix but I agree may be on the lean side for a home brew.
In fact ss needles are a real pain to use and in my opinion only really suitable for a vibrating mix inside a mould.

I did not want to complicate the issue by getting into detail but in recent months I have switched to mixing my own refractory.
I am now using carbon fibres and leave out the needles but still adding polypropylene fibres about 0.1% per mixer load (a loosely filled mug full per load)

The carbon fibres are added in smaller amounts based on the mixes used in rocket stove construction where the temperatures can reach over 1000c .
However I have not tested the mix on a full size dome although the mix is lasting very well on several test beds on other forums.

So yes apologies for quoting misleading info about the quantities of additions to the home brew mix.

[downunderdave]

Thanks for that. Out of interest what are the carbon fibres you use?
I’ve been using a mix of both stainless needles and AR glass fibres in conjunction with castable refractory for some years now and have begun using carbon nano tube reinforcing in both OPC concrete and castable refractory. With good flexural strength improvement. t is fibres on a molecular scale and they also reduce permeability.

[downunderdave]

Aug 20, 2021 8:11:50 GMT lorettaf said:

1. I’m going to build a sand/ powdered fireclay dome. I have available a dumpy bag of what i think is sharp sand from jewsons. It’s grey and is pretty coarse - Is that the right stuff or will it spit grit? Or do i forego my freebie and buy the yellow finer soft sand you see around for play sandpits? Does it matter? I could use the coarse sand for the mold and the rougher cob outer layer.

2. Also I’ve got Eric Wisner’s earth oven book and he suggests 3-5/1 sand/clay slip. The advice on this forum is 2/1 by volume which is a huge difference - any advice? Is it the difference between using dry materials vs dug up clay? I was going to make bricks and dry them. Is 2% shrinkage the max - or too much? Thanks

Pretty much any sand will be ok. If using a cob mix ie sand and clay, there is no cementious material and the resulting mix is not permanent. That is it can return to mud. The addition of cement and lime, both cementious materials creates a stronger and permanent mix when set. The clay content in a cob mix acts as a binder to hold the sand together. If the clay content is too high then shrinkage becomes a problem if too low the clay does not bind so well. Eliminating the water from a cob mix is essential and must be done very slowly to prevent steam spalling. The addition of very fine polypropylene fibres which burn away at low temperature leaving a network of tiny pipes that moisture can move through provides a fair measure of steam spalling reduction.
Different clays have different shrinkage rates ,but an average from plastic state to dry is around 5% with a further 5% if fired in a kiln to earthenware temps. If sand or grog is added to the claythe shrinkage is reduced accordingly. If building a dome in cob over a sand mould the shrinkage needs to be negligible so the casting won’t crack as it shrinks. The removal of the sand once the casting is leatherhard is advised to overcome this problem.

[fox]

Aug 21, 2021 12:02:27 GMT downunderdave said:

Thanks for that. Out of interest what are the carbon fibres you use?
I’ve been using a mix of both stainless needles and AR glass fibres in conjunction with castable refractory for some years now and have begun using carbon nano tube reinforcing in both OPC concrete and castable refractory. With good flexural strength improvement. t is fibres on a molecular scale and they also reduce permeability.

Well we are mainly experimenting with brick size components, the longer the fibres the stronger the result but the more difficult  to mix large quantities (25kg) of refractory mix and fondue cement sílica sand mix.
We have tried cutting the fibres from 100mm to 25mm in length with various amounts of fibres per mix but I am more interested in casting a full size dome.
Carbon fibres dont clump together in the same way as fibergalss as they are much stiffer but the longer the fibre, the more likely to ball up.
We have not found a reliable source of pre cut fibres but it is easy enough to cut up standard weave into the desired length… very much an ongoing project but already there are folk successfully  casting quite large slabs and building forge components.  

[downunderdave]

Aug 21, 2021 17:22:13 GMT fox said:

Aug 21, 2021 12:02:27 GMT downunderdave said:

Thanks for that. Out of interest what are the carbon fibres you use?
I’ve been using a mix of both stainless needles and AR glass fibres in conjunction with castable refractory for some years now and have begun using carbon nano tube reinforcing in both OPC concrete and castable refractory. With good flexural strength improvement. t is fibres on a molecular scale and they also reduce permeability.

Well we are mainly experimenting with brick size components, the longer the fibres the stronger the result but the more difficult  to mix large quantities (25kg) of refractory mix and fondue cement sílica sand mix.
We have tried cutting the fibres from 100mm to 25mm in length with various amounts of fibres per mix but I am more interested in casting a full size dome.
Carbon fibres dont clump together in the same way as fibergalss as they are much stiffer but the longer the fibre, the more likely to ball up.
We have not found a reliable source of pre cut fibres but it is easy enough to cut up standard weave into the desired length… very much an ongoing project but already there are folk successfully  casting quite large slabs and building forge components.  

It is my understanding that normal fibreglass fibres are unsuitable as they react with the cement and corrode. The use of zirconium coated fibreglass (alkaline resistant) gets around this problem, but makes them more expensive. The ones I use are quite stable until the hit around 850C (I've observed a small sample begin to melt in my kiln at that temp). As that is well outside the temperature range of  WFO, they are quite suitable.I thought that the carbon fibres used specifically for concrete are bound up in a polymer which protects the fibres and to use carbon fibres that are designed for glass reinforced plastics in concrete can cause issues.  Is there a corrosion problem using carbon fibres in concrete? And have you tested flexural strength improvement? Also, how do they stand up to heat?

[lorettaf]

Thanks Downunderdave
I’ll definitely have a search for polypropylene fibres and reconsider the homebrew.....
Thanks again, L

[downunderdave]

Aug 23, 2021 7:00:56 GMT lorettaf said:

Thanks Downunderdave
I’ll definitely have a search for polypropylene fibres and reconsider the homebrew.....
Thanks again, L

If you want to research cast home-brew ovens there are a number of well documented builds on the forno bravo forum.