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mackeus 11-01-2009 10:12 AM

Can you make a wood fired oven from an old iron bathtub?
 
I am going to build a wood fired oven next summer. First I planned to start with an earth oven and then I got this hilarious idea about using an upside-down old bathtub instead. I plan to coat the interior with fire clay as you do with wooden stoves. I plan to cover the bathtub with sand as heat magazine. Is it possible?

Will the enamel over the cast iron be poisonous?

/mackeus

RTflorida 11-01-2009 04:16 PM

Re: Can you make a wood fired oven from an old iron bathtub?
 
Sounds interesting. I have four concerns:
1) the enamel, as you mention. I have no idea if it will hold up to the high heat or the thermal cycling of an oven, I know indoor gas and electric ovens have enamel finishes, but they typically only heat to 500-550 F. A wood fired oven reaches double that 2) Getting your fireclay mixture to adhere to the enamel. 3) The thermal expansion properties are vastly different between your fireclay mixture and the (cast iron?) bathtub. If you didn't have the adhesion problems listed in #2 during application of the fireclay, you will after a few thermal cycles. 4) Your use of sand, a proven NON insulator. Even if everything else works perfectly, you will have an oven that won't heat up enough and will not retain heat.

Have you downloaded the free plans from the FB store? If not, you need to start there; regardless of the actual oven type or alternative materials you may choose, there are "basics" that every oven build should follow - PLENTY of insulation both below the hearth and around the dome (oven chamber/bathtub) being the most critical.

RT

ThisOldGarageNJ 11-01-2009 04:54 PM

Re: Can you make a wood fired oven from an old iron bathtub?
 
if your serious about the tub,, have a bigggggg fire in it now right side up.... see what happens to the enamel...

I personally wouldnt, (as RT said)as being that old you cant be sure what additives are in the enamel ie:lead... You could always make a hillbilly hot tub out of it

nissanneill 11-01-2009 08:29 PM

Re: Can you make a wood fired oven from an old iron bathtub?
 
I don't know about the enamels used on such gear. You still see enamelled frying pans and pots still being used over gas, electric or wood fires without problems!
A research exercise should give some idea to their composition, but if you want to get a tub re-enamelled, it is subjected to temperatures in the vacinity of 1000˚C.
It certainly makes one think about alternatives somewhat!
Cutting and then joining it will sort the men from the boys eh? Welding then together without crystallizing the metal, cutting your entrance and then the adding of thermal mass also makes one think.
I can't see it impossible, just a different set of hurdles to overcome.
Maybe you want a long thin oven rather than a round one.

Neill

Wiley 11-02-2009 10:05 AM

Re: Can you make a wood fired oven from an old iron bathtub?
 
I used to do a fair amount of enameling and so have a bit of knowledge on this.

First off, depending upon the company who manufactured the tub and the year it was manufactured the composition of the enamel will vary. Older tubs were covered with a lower temperature "soda based" enamel. These enamels needed lower temperatures with which to fuse and often did not require a base coat as do more modern enamels. More modern tubs use higher temperature more chemical resistant enamels.

To check if there is lead in your enamel and this is a direct quote from "Fortunes in Formulas" 1944:

"Destroy the outside coating of the enamel at some spot by the application of strong nitric acid. Wash the part and apply a drop of ammonium sulphide. If lead is present, the part will become almost black, but remains unchanged in color if it is absent."

If you are unable to obtain the nitric acid, you might have luck simply filing away a bit of the surface glaze (shiny bit) with a diamond file to expose the interior of the enamel and then apply the ammonium sulphide. Since the directions are saying to wash the part affected by the nitric acid one would think that the nitric acid itself is playing no part in the chemical reaction.

So you will need access to a couple of small amounts of some chemicals. Nitric acid is nasty stuff so don't get it on your skin or in your eyes. You will only need a few drops and the gov't will be interested in talking to you if you are trying to acquire larger quantites. You might try your local high school chemistry dept. Tell them what you are wanting to do and I suspect they will be of help with the quanities needed of both chemicals.

Otherwise be aware that most enamels fire (fuse) at relatively low temperatures. I have enamels that fuse below 1000 F and they range upward to around 1900F. The variation is so that one can apply layers and refire without disturbing the previous firing. So again depending upon the age of the tub you may or may not be successful in using it as a interior for a WFO.

As to welding, good luck as I suspect you will be unsuccessful. The enamel makes fusing of the metals difficult. Steels and iron are porus (especially cast iron) to enamel. It is hard for some to believe but one can make a pattern on one side of a sheet of rolled steel with enamel and fire it and it will show on the other side. I've done this with sheets as thick as 1/16th inch and cast items are very porus...that's why the outsides show very little suface rusting even after being set out in farmers fields as water troughs for years and years.

And finally, if you have a short tub (like the 4 ft models made) they are very sought after and even if the enamel is damaged are worth alot more than one might think. Otherwise the 5 footers and up are not uncommon and can be fairly easily obtained.

Hope this helps,

Wiley


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