|Robin Graham||25/03/2018 22:14:29|
|612 forum posts|
I'm planning on making a furnace along the lines of that described by Michael Cox in MEW 181 and on his website **LINK** .
Michael's recipe for the castable refractory calls for a mix of Perlite, clay (cat litter) and Portland Cement in dry volume ratio 7:1:1.
There was a brief discussion of Michael's recipe on this forum (**LINK** ) in which one respondent (Vic) reports that the refractory (made to Michaels's recipe 'or something similar' ) became fragile after a few firings. It may be that the devil is in the 'something similar' of course, but I have read elsewhere that Portland cement is less than ideal as a binder for refractories. In the same thread Martin Kyte recommends Ciment Fondu as a binder, and vermiculite as filler, but doesn't give a recipe.
So, to my questions at last - first, does anyone have experience of using Ciment Fondu for this sort of thing, and second, is there any reason for preferring Perlite over Vermiculite or vice versa? In his MEW article Michael specifically advises Perlite rather than Vermiculite, but doesn't explain why.
Edited By Robin Graham on 25/03/2018 22:15:38
|Nick Hulme||25/03/2018 22:33:07|
|711 forum posts|
When I needed a castable refractory for a project I bought 10Kg form a refractory supply company, I can't remember the price but remember thinking it was surprisingly reasonably priced, it worked nicely and is still holding up.
|norman valentine||25/03/2018 22:41:57|
|209 forum posts|
I agree with Nick, I bought the right stuff and my foundry is in its sixth year and nothing flaking off.
|1364 forum posts|
Do you have the Supplier & Product details that you would recommend Nick/Norman?
|Michael Cox 1||26/03/2018 11:05:45|
|519 forum posts|
My furnace still has the same refractory lining that I made for the description given in MEW 181. The refractory is low density giving excellent insulation. The furnace has probably been used at least 50 times since the original refractory was cast in position and it is still sound. It is important to dry the refractory slowly to get a strong lining.
On the subject of perlite versus vermiculite my understanding is that perlite is firstly more heat resistant that vermiculite and secondly vermiculite will take up water from the atmosphere. This means that whenever the furnace is used the adsorbed water in the vermiculaite has to be driven out of the refractory.
Portland cement on its own does not have good heat resistance and thermal properties which is why more clay was added to the mix.
High alumina cement (ciment fondu) would be a good alternative to the portland ciment/clay mix but it is not as readily available. I tried to make my furnace using readily available materials.
|Martin Kyte||26/03/2018 12:34:23|
|1515 forum posts|
Not sure you need a recipe. Supplies of ciment fondu are readily available from ceramics suppliers (just google it)
Follow the instructions on the bag and add as much vermiculite (or perlite if you prefer) as you feel you can get away with. It's ages since I did it so I cannot help much more. Two of us built a gas fired pottery kiln at sixth form so that where my expirience came from.
4796 forum posts
A lot of furnace recipes use fire cement. It has different properties to Portland and has to b fired to bring them out. readily available in places like B&Q in pre-mixed tubs so a bit more difficult to work into the mix. A powdered version must be available.
|Split Pin||26/03/2018 13:32:52|
|9 forum posts|
Many moons ago I worked as a maintenance engineer on plant which was used for the manufacture of Castable Concrete products suitable for temperatures up to 1850C. Fondu was the cement portion used up to around 1600C
938 forum posts
I have used cement as the main binder in the lining for my furnace, melting aluminium. On occasions I have slightly melted the lining and now, after 15 years, the base is close to dropping out. As the fuel for mine is pallets, e bit of extra heat loss is not too much of a problem & the cost of the lining is tiny.
On the cat litter side, Asda 'Smart Price' clay litter is about 50% gravel. The Wilkinson's (Wilko) stuff is almost 100% clay. Found that out this January when tempering some sand for the moulds.
|jaCK Hobson||26/03/2018 16:24:46|
|164 forum posts|
I think fresh vermiculite works OK and also think damp was the stated reason not to use it. I made some with perlite and melted it at sustained steel welding temps and it will dissolve quickly if you use borax flux. OK for forging temps or melting aluminium. Probably not going to support making crucible steel or cast iron. I use the proper stuff now - postage is the killer cost
|Rik Shaw||26/03/2018 17:05:09|
1313 forum posts
Many years ago a friend and I attempted to cast cylinders for SIMPLEX. We made a melting pot from a bit of large dia. metal pipe with a plate welded on one end. We just lined it with ordinary fire clay. It worked and melted the bronze OK. Just a pity we made it to small so there was not enough "melt" for a complete cylinder.
|Robin Graham||26/03/2018 20:58:22|
|612 forum posts|
Thanks for all your replies, and especially to Mike for giving the thinking behind his formula and an update on the performance of the furnace described in his article. Apart from being more readily available, Portland is also a lot cheaper than ciment fondu ( around 4 quid for a 25kg bag against ~30 for fondu), and clay litter from Wilko (I lucked out there, I'd got some Wilko stuff, thanks for the info Richard!) is about £1 for an 8 litre bag. So way to go I think - I shall certainly experiment with this mix.
A couple of replies referred to the 'right' or 'proper' stuff without saying what that actually is - like IanT I'd be interested to know!
|jaCK Hobson||27/03/2018 16:56:27|
|164 forum posts|
Put 'Castable refractory' into google and it is easy to find a choice of readymixed products with different temp ratings although you get quite a bit of noise about pizza ovens.
Trouble is, there is often a min £50 delivery charge for a pallet, and you have to be available when it is delivered.
|Les Jones 1||27/03/2018 17:16:44|
|2097 forum posts|
I have built a small furnace for casting aluminium using a simialr mix to the one Mike Cox used. It has much better insulating properties than the castable refractory that I use to build a larger furnace. The liner made from castable refractory needs insulation around it to reduce heat loss so you need to consider more than just the cost of the castable refractory. You will also need fibre insulation and insulating fire bricks . This is where I bought the materials for may large furnace. I collected the items so saving transport costs.
Edited By Les Jones 1 on 27/03/2018 17:18:17
|norman valentine||27/03/2018 18:19:22|
|209 forum posts|
I am afraid that I have forgotten where I bought mine, all I can remember is that it was a company in Nottingham. They sold refractory bricks to industry.
Sorry that I can't be more help.
|Robin Graham||28/03/2018 22:59:38|
|612 forum posts|
Thanks for further replies - my mental fog is clearing! The Vitcas website which jaCK mentions is very informative and the artisanfoundry site which Les gave a link to is deffo worth a look for anyone contemplating home foundry stuff.
What I'm after is a lightweight insulating material rather than the dense high thermal mass stuff which seems to be the best choice for 'proper' furnaces - eg pottery kilns, The thing is going to be a bit bigger than Mike's furnace - the outer case is one of those garden incinerator bins - so maybe about 60 litres of insulation, which would be quite heavy in solid fireclay, and take an age to heat up I guess. So I'll go for the perlite/cat litter/Portland mix.
If all goes well this will double as a tandoor . Mmmm, tandoori chicken... erm molten aluminium I mean, sorry, senior moment there.
4796 forum posts
The 'castable refactory' is indeed heavyweight and is intended for the half inch thick layer in contact with the flame or to be mixed with your pearlite to form the backing higher insulating layer.
|2332 forum posts|
The last time I looked Perlite and Vermiculite were about the same price at the Garden centre so I’d stick with the Perlite most recipes specify. The Bentonite cat litter (clearly marked on the sack) was either Tesco or Sainsbury’s.
|jaCK Hobson||29/03/2018 08:31:33|
|164 forum posts|
Mixing the stuff is a bit of a bother. First you have to get the cat litter turned into clay. I did a bucket at a time and used a paster/big paint mixer on a big (cheap) sds drill with all the final amount of water (takes experimenting) . Then mixed with cement in a wheel barrow and a hoe, then perlite. Easier to do small batches than one big batch.
|not done it yet||29/03/2018 11:00:47|
|3576 forum posts|
Portland cement is not a good choice for durability. Concrete cracks and spalls and portland cement loses all strength on heating to about 900 Celsius. We did that to concrete samples when analysing the make-up.
Portland cement strength comes from hyrated silicates. Dehydration occurs anything above about 600 Celsius IIRC.
Calcium carbonate loses CO2 around 900 and Portland cement will melt at about 1500, dependent on composition (ignoring the gypsum content).
Most certainly, some form of insulating layer in any furnace is good for energy conservation and so speed of melting the bit, that is needed to be heated, is accelerated. Rotary cement kilns used to have two layers of refractory in some regions of the tube - one for abrasion and chemical resistance and the outer layer as insulation.
Now, melting Rhodium/Platinum mixtures is another matter. We used an induction furnace and carbon crucibles for really high temperatures. But that was thirty years ago.... so doubtless things have changed a bit, since then.
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