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-   -   Help I used the wrong castable....suggestions? (http://www.fornobravo.com/forum/f6/help-i-used-wrong-castable-suggestions-6597.html)

Bill1 04-19-2009 06:24 PM

Help I used the wrong castable....suggestions?
 
Hey all

Great site, just wished I'd found it before I started work on my oven!

At this stage I have cast my dome over a sand mould. It was only after doing this that I realised from reading the posts on castable refractory cement that I have used the wrong product. I cast it instead of bricking for budget reasons- as you may be aware firebricks are not cheap here in Western Australia.

The product I used is a medium weight insulating castable called Shiralite 130MW. Will paste some data below for those of you who understand it.

The reason I used it was that the refractory stockist advised that it was the right product to use and that it was used by local commercial kit makers. Given my reading from this site I am now rather unimpressed with that advice :mad:

My questions are these: Will it still work as an oven- what is the effect of using the insulating castable ( I had hoped to use it to make bread and other slow cook items so am wondering about heat retention)? Should I knock it down and start again? (cant really afford to :( )Or is there something I could do to improve its performance such as render the inside of the castable with non insulating refractory cement?

I had been planning to use ceramic insulation and then vermiculite render on top of the dome I had cast but it would seem that my oven may be insulated enough....

Any suggestions would be much appreciated. Yes I'm an idiot for not reading enough before I started...

Shiralite 130MW
TYPICAL PROPERTIES

Al2O3 ......................................... 46
SiO2........................................... 38
Fe2O3......................................... 1.5
CaO........................................... 11
TiO2........................................... 1.4
MgO........................................... 1.1
Alkalies...................................... 0.6

Maximum Service Temperature (oC): 1300
Nominal Shelf Life: 12 months

Approximate Thermal Conductivity:
oC 250 500 750 1000 1250
W/mK 0.40 0.42 0.44 0.46 0.48

Installation Procedure................... IP/001, 002
Heat Up Schedule........................ HS/002

Bulk Density (kg/m3)
After Heating to 110oC.................. 1550 - 1610
After Firing to 1000oC................... 1440 - 1500
After Firing to 1200oC................... 1560 - 1620
Cold Crushing Strength (MPa)
After Heating to 110oC.................. 5.5 8.0
After Firing to 1000oC................... 3.0 5.0
After Firing to 1200oC................... 3.0 5.0
Modulus of Rupture (MPa)
After Heating to 110oC.................. 2.7 - 6.0
After Firing to 1000oC................... 1.0 - 4.3
After Firing to 1200oC................... 1.9 - 5.2
Permanent Linear Change (%)
After Heating for 24 hrs at 110oC..... 0.0 to -0.2
After Firing for 5 hrs at 1000oC........ -0.2 to -0.5
After Firing for 5 hrs at 1200oC........ -0.5 to -1.5
Maximum Grain Size (mm) 4

jpar 04-20-2009 01:47 PM

Re: Help I used the wrong castable....suggestions?
 
The material is a little lighter then what we have had success with, but its not to far off.
I think it is worth keeping intact. The lower density will allow the oven to heat up a little faster, but is will aslo cool down faster because of the lower mass

james 04-20-2009 02:12 PM

Re: Help I used the wrong castable....suggestions?
 
That is pretty light. I converted 1500 kg/m3 and got about 93 lbs/cubic foot. Lighter than you would want.

Does anyone else want to jump in here?

I would want to think about the oven's heat holding capability. Is there any way of testing it before you go all the way through to complete it?

James

dmun 04-20-2009 04:42 PM

Re: Help I used the wrong castable....suggestions?
 
Piggybacking on James estimate, a firebrick is roughly 72 cu in, or about 1/24 of a 1728 cubic inches in a cubic foot. 93 Pounds divided by 24 is 3.875 pounds, or about half what you would expect a firebrick to weigh. Sounds like an insulating material.

Since it's already built, I'd say try it with fire-in-the-oven pizza making. It may work great, let us know. For retained heat bread baking and roasting, I kind of doubt it will work.

Why, oh why doesn't someone go into the discount firebrick business in Australia? Are there ruinous import duties or something? Surely someone in the import/export trade could do this as a profitable sideline at almost no cost. It seems that you could keystone firebricks, and still underprice the domestic prices by half.

Bill1 04-20-2009 05:24 PM

Re: Help I used the wrong castable....suggestions?
 
Thanks for the replies guys.

At this stage I can test it as is before I add the insulation (ha didnt I already do that :) ) and flue.

My oven is reasonably large (internal diameter of 1.2m or 47") so I had thought that if it is really bad I could stack up some firebricks around the inside for baking puposes. I only really need enough space for a few loaves of bread or a largish piece of meat in the middle so adding thermal mass as and when I need it might be a good solution.

I used the inch thick firebricks (tiles) as a floor so there wont be too much heat retention there either I guess)

I think that I will probably only be using the retained heat aspect of the oven maybe 15% of the time, so maybe its not too much of an issue.

For pizza purposes the quick heat up time will be useful though I suppose.

I guess making sure I do a good job on the top layers of insulation will make sure as much of the heat as possible stays in. Was going to go with 2inches of ceramic blanket followed by vermiculite render and waterproof layer. Is this worthwhile or will I be overinsulating then?
Am thinking that the door will also need to be fairly well insulated.

Thanks again for your time.

dmun 04-20-2009 05:40 PM

Re: Help I used the wrong castable....suggestions?
 
The conventional wisdom around here is that you can't be too thin, too rich, or have too much insulation. :)

But hey, try it the way it is, first. If it's only warm to the touch with a raging fire inside, you have all the insulation you need.

Bill1 04-20-2009 05:57 PM

Re: Help I used the wrong castable....suggestions?
 
Thanks Dmun

Have to go through the curing process yet but will let you all know how I go.

Here's hoping!

Bill1 04-28-2009 05:34 PM

Re: Help I used the wrong castable....suggestions?
 
Update

Well the curing of the internal shell is finished. I got a series of hairline fractures (which I was told were unavoidable-and are only open while it's hot) but the dome seems to have maintained structural integrity.

In terms of the shell heating up, on the final curing fire the whole structure was so hot I could barely touch it (1 and a half hours after lighting). So I guess that although the castable has insulating qualities it seems to be heating up appropriately.

I am a bit concerned about whether it is reaching the right temperature though. The inside didnt turn white as I have seen with other ovens in terms of indicating cooking temperature. Can anyone advise whether this is likely to happen before the internal layer is insulated or is this an indication that the castable is not functioning correctly or that the measurements in terms of opening to dome (currently at 63% of dome height)??

brickie in oz 04-29-2009 02:15 AM

Re: Help I used the wrong castable....suggestions?
 
Quote:

Originally Posted by dmun (Post 54166)

Why, oh why doesn't someone go into the discount firebrick business in Australia?

Plenty of stuff for sale at reasonable prices here.
refractory and ceramic
And here.
Darley Refractories Australia, Bacchus Marsh, Victoria

david s 04-29-2009 04:48 AM

Re: Help I used the wrong castable....suggestions?
 
The castable refractory I use is around 2000 kg/cu m so yours at 1500 is a bit lighter but should probably still work ok. You'll have to go through several firings to eliminate all the water. Don't be in a rush to get the dome white yet. Read the sections on curing in the forum.


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