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Thread: Clay Crucibles and design for wootz/poulad

  1. #1
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    Clay Crucibles and design for wootz/poulad

    Hi all

    I think since we have some successful crucible making going on... that this is a topic really worth discussing !
    -- since wootz making revolves around successful crucible technology.. its important to have a discussion on methods...

    I've had very poor success when it comes to crucibles and have opted to buy commercial clay graphite crucibles, instead.. .... but i would like to come back to making my own

    firstly... how did the ancients do their crucibles..

    here is a study on it by T L Lowe.
    conference " Principles of solidification and materials processing: proceedings of Indo-US workshops "
    1990 pg 729-740
    " Solidification and the Crucible processing of Decanni Ancient Steel "

    the diagram is a crucible with description


    Composition of cover fabric:
    1) glassy matrix- coarse and fine vesicular low Fe glass
    2) fabric componenets-recycled fired vessel refractory fragments, quartz and feldspar coarse sand, clay
    3) numerous micro and macro iron prills in the glassy matrix

    exterior surface:
    4) coarse quartz and feldspar sand in ash glaze matrix, patches of red iron oxide dust
    5)prominent pinch marks from tongs

    interior surface
    6) reaction surface of base of cover with hundreds of small iron prills
    7)interior wall glaze
    8)slag fin left when ingot removed
    9)slag glaze in which ingot rested

    Composition of refractory fabric
    10) coked rice husks in finely vesicular glassy matrix


    this is a very good study if you can get your hands on it..... I believe there are more studies out there.... if you know of any... please help list it

    awhile back I posted this on crucibles aswell...

    http://p222.ezboard.com/fprimalfires...icID=219.topic


    take care
    Greg

  2. #2
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    Greg,
    I spoke to Thelma many years ago about this and my fists wootz smelts (what 13-14 years ago?) were from crucibles made to this design. The wall thickness needs to be addresed as it takes quite a while for the heat to penetrate. It is MUCH slower than modern crucibles, but then again the old ones can take quite a bit of abuse in the fire while hot.

    One of the things is to fold in the coked rice hulls while dry (dry mix just like in baking) and slowly add water. Let the mixture sit after folding in the water (just like baking). Allow a lot of dry time (five days or so) before cooking the crucible at a low heat and then you can charge them with metal and fire a smelt.
    It take a while, but the materials are inexpensive. The folding allows for large rice hull parts to help with dimmensional stability of the crucible at temp.
    The crucible technology is as interesting as the metal and as advanced given the time frame of the technology, but most only care about the metal.


    Ric
    Richard Furrer
    Sturgeon Bay, Wisconsin
    http://doorcountyforgeworks.com/

  3. #3
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    Greg and Richard, thank you very much for pointing this out. Interesting thread.

    Kind regards

    Manouchehr

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    Greg, please explain 'coked rice hulls' . While organic matter is used in clay for molds [Jeroen has strange tastes and uses horse manure !! HAHA ] is the coked rice hulls coked [turned into charcoal ?] or does it turn that way in the smelting process !!

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    thank you Manoucher


    Hi Robert

    i've re-read parts of the study... and it definitely says that the rice hulls are charred prior to mixing with the clay

    It also says in the " Voysey" account that the rice hulls and clay were mixed with oil.. ... it mentions that during ignition, the surface s of the polysilicic form of silica become partially dehydrated and are more compatible with the oils than with water..

    interesting stuff in this study...

    i had another study.. that said the carbon in the hulls produced some Silicon carbide nodules in the refractory.. .. ( not sure where that paper went )


    Hi Ric

    thanks for the tips.. the crucible does look rather robust !
    I wish i had your contacts... thats great that you talked to Thelma ..
    wootz from around the globe

    now i have so much catch up to do... this is one area where i really did poorly

    take care
    Greg

  6. #6
    Interesting side note on crucibles and the crucible's effect on the melt:

    One of the big worries in steel making in general and wootz making in particular is excess sulfur. Most modern crucibles and all of the ancient crucibles that I have ever heard of have been made from silica based materials such as fireclay. I am excluding the silicon carbide and clay-graphite crucibles because I believe they are beyond the production capabilities of the average blacksmith's shop. Silica is regarded as an acid at all temperature ranges, as is sulfur (and phosphorous for that matter). Therefore the two do not react with each other in during a melt. Similarly, a glass slag will not react with sulfur. One of the traditional ingredients in a wootz melt was crushed seashells or some other source of calcium. Calcium is a base, and therefore will react with the sulfur if given enough time, but it will also react with silica in the walls of the crucible, shortening its lifespan and also rendering the calcium slag itself less effective. In industry, they solve this problem by making the walls of most open hearth furnaces from basic materials such as dolomite, or even better, magnesia. Limestone can then be placed safely in the bottom of the furnace prior to loading the charge for the melt, and in the later stages of the melt will float to the top of the molten steel, hopefully reacting with the sulfur and other impurities and carrying them into the slag. I guess the big question here is, does anyone know of a source for magnesia crucibles or rammable refractory to make crucibles from? In theory it should make home melts much cleaner.

    Peter
    http://www.fallinghammerproductions.com
    http://www.dragonsbreathforge.com/wootz.htm

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    Greg, Rick, Peter:

    Thanks! Wow, when it rains it pours.

    Robert,
    Rice hulls are sold here in CA in big bales for about $6.00 per 40 lbs.
    They are used by horse owners. By just placing them on a sheet of iron and roasting them they will char nicely, the smoke is a killer to breathe and get out of your clothes.

    Peter,
    Thanks on the information on a W-2 source (do W-1,1095 and W-2 all have the same spark test?).
    The carbon level of the four samples I listed regarding the Sulphur content are not the same metal I will be placing in the crucibles (though there is no reason not to). I will be starting with cast iron (home made form magnetic sand and charcoal in a tatara style furnace). So carbon pickup will not be a problem.

    Thick insulating walls (3/8", lots of rice hull char) can be a problem in getting the charge up to temperature (this may be a cultural problem only).
    Somewhere I read, the rice hulls create a reducing atmosphere within the body of the clay, this in turn converts iron oxides (frequently found in clays) to iron. If the iron oxide were not reduced, it would act as a powerful flux and the crucible would fail. Now the outside glazes over (which is good, it keeps the oxygen out), the iron oxides have been reduced to iron and the inside of the crucible has a reducing atmosphere and hopefully very little iron oxide in the charge.

    As to the shape of the crucible I am going to stay with the basic cup shape or roughly an assay crucible shape until I learn more about the clay and the recipes.

    Regarding making a clean refractory, I think it is easier to work with clean metal. If clean cast iron is hard to get why not work with " pure iron"
    from Austria, someone in the US is a distributor. Instead of adding Carbon via cast iron you could add it via charcoal or wood.

    I have a crucible (made this evening) drying. It has the following composition:
    5.5 lbs of grog (AP Green)
    1. Tenn. ball clay
    1. EPK (extra plastic kaolin)
    2. Lincoln 60 fire clay
    .3 Feldspar (potassium)
    1. Rice straw dust (not charred)

    I thought I would test one or two crucibles and adjust the batch if needed. Today I put the chainsaw to a bale of rice straw to make some horse dung without the odor, swept up the dust, sifted it several times.

    The rice straw (or hulls) is used for the iron making furnace clay, I prefer shorter straw fibers over long ones.

    The A.P.Green fire clay grog has been my baggage for 35 years, I do not think I can get it anymore (back the crushing soft fire bricks).

    Jan

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    Hi All,
    Bronson's paper (Archaeomaterials, 1986) gives a summary of the different crucible shapes and compositions, as well as furnace designs and crucible charges based on ethnographic evidence. Just some differences to ponder...in Central Asia we have high quality refractory clay being used along with grog in the Merv crucibles and quartz fragments in the Uzbek ones. In India it is usually ordinary clay and rice husks, but the Hyderabad process is a mixture of technology. There are quite alot of similarities (considering a ~ 900 year time difference) between the Hyderabad material (as published by Lowe, and discussed by Voysey) and the Merv material, such as the use/recycling of old crucibles in the furnace walls, flat bottomed crucibles, refractory clay for the crucibles. But there are also differences such as the use of rice husks. Voysey mentions that Persians were in charge of the process in Hyderabad, perhaps this is evidence of Persian/Central Asian methods of crucible steel making being "reintroduced" into India. The "Hyderabad process" is a real mixture of South Indian and Central Asian technology. FYI, I am constructing a (hope to be) comprehensice list of all textual, ethnographic, and archaeological evidence of the manufacture of crucibles and charges known so far.
    Dr. Ann

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    Hi Ann

    I've read the post twice... excellent info in there.. the contrast in crucible technology between regions is wonderful.. ..
    --
    -- I can see that a list of all crucibles and charges would be a very very valuable resource !
    -- oh.. and any estimates or eye witness accounts of firing times..

    Greg

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    Hi Greg,
    Yes, firing times vary between 1 hour to 25, cooling rates vary from cooled with water while still hot to left in furnace to cool. Also, number of crucibles in furnace vary from 1 to 59. Lots of variations!
    Dr. Ann

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    This evening I have had another opportunity to test a clay crucible. Though again it failed, a lot was learned, (again).

    The crucibles made with the mix mentioned in an above post were dried and fired, to dive off the chemically bound moisture, over a wood fire. The fire was blown with air to create visibly red (in daylight) crucibles. All but one crucible had a clear bell type ring when struck and appeared to be sound.

    The crucible used did not have a good tone and had some small cracks at the top rim ( a thicker than needed rim). The defective area was cut off with a zip blade...after which the tone became sharp and clear.

    The crucible was filled with broken file bits to the very top (packed) , then covered with brown paper which was sealed with wax (to avoid bonding of metal with the lid material) . The lid was rammable refractory , four small holes were punched in the top.

    Preheated the furnace with a small propane torch for 1.5 hrs (until rammable was dry), then at red heat for 20 min. and at full heat within .5 hrs. At 1.0 hr. into the heating I stopped due to a very hot spot at the bottom (exterior of the furnace). Oh oh.

    Probable cause of failure is:

    The crucible had not yet done any shrinking even though all the chemically bound moisture had been driven off. The tight packing of metal resisted the shrinking of the crucible and it broke well before the metal was melted. I have added some pictures of the results and will try again in a day or two to confirm my suspicions.

    The pictures are of:
    1) The crucibles after low firing
    2) The crucible packed with metal
    3) The lid in place
    4) Furnace
    5) Crucible remains

    Jan
    Attached Images Attached Images      

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    Hi Jan
    thanks for sharing that... and your firing specifications... that really is the key to moving forward..

    well.. one thing you could also do... is smash that crucible up and use it for the grog of the next one.... i know the ancient crucible had that put into the mix..

    recycling ..

    i think the reason for grog is for less shrinkage... since its already fired.... but i could be wrong on that


    Greg

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    Hi Jan and Greg,
    Some tidbits...it appears at Merv they did not prefire the crucibles, just air dry to a leather hard state. Yes, I know that is conterintuitive, but isn't all things related to wootz/pulad? Grog does prevent some shrinkage but also (yes recycling so less clay) but grog and other inclusions such as quartz prevent cracks from spreading (as does voids left by the disintergration of rice husks), gives better texture, hardness, strength, etc.
    Dr. Ann

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    I would have assumed that prefiring would not only shrink the crucible but there would be the bonding [stronger] of the particles . "it's a puzzlement " as the man said !...Ceramics certainly don't like thermal shock or thermal fatigue.

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    Dear Greg, Ann and Robert,

    I am not discouraged, and think the clay mix will hold up to the required task. This now iron laden crucible would better be left for furnace building grog. Actually it is my first ceramic sculpture.
    The crucibles (some of them) were able to be placed on top of our wood stove while at a soft leather hardness and were dry in another 24 hrs..They were placed in the flames of a slow burning fire so in all they were very porous and easily lost the "pore" moisture.

    The chemically bound moisture is lost between 650 Deg F and 950 Deg F. At this point the crucible is not very strong and is about the same size as it was after drying. The wall looks very much like insulating brick (the pores of the crucible are smaller, but the clay feels stronger).

    One of the accounts of early wootz making, mentions the crucible being placed on the coals but no serious blowing is done until they stop evolving gas. This seems to do what I did but in one step.

    However after firing at about 2600 Deg F? for .5 hrs a lot of shrinkage was evident. The clay was glassy ,fewer and smaller pores, very strong, and had a really crisp ring to it. I would say the wall shrunk by 33% in thickness.

    So: Grog= Stronger, open and more refractory
    Straw dust = Open, insulating and seems to aid the handling of the
    plastic clay

    The cleanup of the ever growing junk pile (my materials resource center)
    is ongoing, but for some reason (and now I know why....because crucibles shrink) I have never been able to dispose of the piece of steel shown below.
    I am sure you understand.

    Jan
    Attached Images Attached Images  

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    Hi Jan

    excellent ... information...

    now you mentioned some shrinking going on... hmm.. that may explain something i wondered about...

    in Brian Gilmour and J Allen's book... persian steel the tanavoli collection... there is a picture of an old crucible and its charge...
    -- it looks like the charge is made narrow.. ... maybe for the shrinking that goes on..



    by the way... that is a good book to read...

    also there is a diagram of a Merv crucible aswell.... very nice..

    Greg

  17. #17
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    FYI, I am constructing a (hope to be) comprehensice list of all textual, ethnographic, and archaeological evidence of the manufacture of crucibles and charges known so far.
    Dr. Ann, will you be posting the list hereabouts? And especially, will you post links to any online references?

    This is a great thread. Has there been a pointer to it from the metallurgy forum? I know several of the regulars there will want to read this, especially Jerry Bennett...

    {edit} I went over and added a cross reference there. I think lots of the metal heads will want to read this information.
    Last edited by jim frank; 01-30-2007 at 10:39 AM. Reason: to add info
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