401 crucible furnaces

8/13/2019 401 Crucible Furnaces

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Crucibles and their Furnaces


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Crucibles A crucible is a container that can

withstand the very high temperaturesnecessary to melt glass, metal,pigments and some laboratory

materials. Bronze and Iron age crucibles were

shallow clay vessels and the air forthe heat was blown from above. Theyoften had small handles to aid in

transport from the furnace to themold In Roman times, due to advances in

furnace technology (furnacesswitched to heat from below), the

crucibles changed to rounded orpointed bottomed conical vessels.The also began using cementation(the process of mixing copper withzinc oxide or carbonate the zinc

vaporizes and binds with the copperto make brass).

Iron Age crucible fromLlwyn Bryn-dinas hillfort,Llangedwyn, Wales, UK


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Crucibles In the Mid-evil period, crucibles developed a

flat bottom and thinner walls. They alsobecame larger.

The Roman cementation process was lostuntil the late middle ages and continued upthrough the 19 th century when anothercementation process was discovered: steel(although steel through cementation processwas performed in India around 300 BC and iscommonly known as Damascus Steel butmore on that later)

In the post Mid-evil period two new type ofcrucibles were introduced. The Hessencrucible was by far the most successful andmade of a high alumina Kaolin clay which wasthrown on the wheel and fired in a kiln. Thistype of clay was not formally discovereduntil the 20 th century when mullite which is

crystallised from the decomposition ofaluminium-rich kaolinite clay at temperaturesover 1100C. This material is found inairplane engines and other high temperaturelocations

The second type of crucible also came fromGermany and was made primarily of graphite.

We still use similar crucibles in foundriestoday.

Hessen Crucibles


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Modern Crucibles Crucibles are numbered accordingto the amount of aluminum they

will hold. A number six crucible isabout 6.5 tall and 5.25 indiameter. It will hold 6 pounds ofaluminum or 18 pounds ofbronze/brass.

For small amounts of lowtemperature metals you actuallycan use a heavy steel or castcontainer (1/4 thick walls).

Ceramic crucibles are lessexpensive but not very durable.You do not want to use a largeceramic crucibles, for this reason.


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Crucibles

Graphite crucibles arerelatively inexpensive andused by many foundries insizes up to 60. However theyare fragile and do not havevery long lives. (top image)

Silicon carbide crucibles arethe most durable and longlasting. They will handle themost abuse and what Irecommend. They are themost expensive of thecrucibles, but will last formany years with care.(bottom)


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Care for your crucibles Crucibles must always be annealed on 1st use. They occasionally need to be re-annealed, by returning to

the furnace empty and taking it up to 2000 degrees, thenallowing to cool slowly.

They should never be over packed with cold metal (metalexpands when melting).

They should be heated slowly until the furnace gains color.

They should be cleaned at the end of the melt. They should always cool in a warm furnace and never

allowed to cool in ambient temperatures. Store your crucibles on wood blocks or upside-down. Do

not store them in places where they will get wet or freeze. The sides and lips should be dressed regularly. Always examine your crucible carefully before each melt.

The ring test should help you find cracks. NEVER EVER USE A CRUCIBLE THAT LOOKS LIKE

THESE! METAL CASTING IS DANGEROUS! ANDTHE PRICE OF A NEW CRUCIBLE IS LESS THAN THE

PRICE OF A TRIP TO THE ER!!!


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Crucible TongsTongs should allow you to lift the crucible easily from the furnace. Theyshould also cradle the crucible gently, it should not rock back and forth, itshould not pinch the crucible.


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Crucible furnace design basics Round furnaces are more efficient than

square furnaces. Furnaces should be sized according to

the number crucible you are using. They should be 6- 12 wider than the

diameter of your crucible. The furnace height, with liner, should

account for a crucible block which getsthe crucible above the force of theflame and allow 1- 2 of space betweenthe lip of the crucible and the lid.

The burner port or tueyer (tweer)should be placed below the bottom ofthe crucible and be tangent to the sideof the furnace liner.

Depending on the metal you aremelting and the liner you are using,

allow for a liner thickness of 3- 6


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Liner for the furnace The best liners are pre-cast bricks made of alumina ceramic or silicon carbide.

If you buy a furnace, it will usually have this type of liner. Otherwise you will cast or ram your liner which will be made from a high

temperature refractory usually a clay or cement. There are a great number of refractories on the market. Be sure the material

you are using will work for the temperatures and metals you are using. The choice between castable or ramable is a highly personal one. I prefer

ramable for the control, although castable produce nicer surfaces and arefaster to form

My favorite is a product called Meltzona that comes from Alabama, it isinexpensive, withstands the temperatures of iron and fluxes over instead offluxing out.

After you have cast or rammed your liner, you want to let is sit overnighttightly wrapped in plastic and then fire it in slowly the next day.

It is very important not to let surface of the refractory dry out or it may skinover and trap moisture inside. When this liner is fired it is very likely to crack.


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Charcoal Crucible Furnace Charcoal fired crucible furnaces

have been used for thousandsof years and run on similarprinciples as modern furnaces.

Essentially you have a crucible

which is surrounded by fuel(charcoal, coal or coke) andaccelerated by the addition offorced air.

Two of the biggest changes intechnology were making thefurnace cylindrical and havingthe forced air come in the fromthe bottom.


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Running a Charcoal furnace Charcoal fired furnaces are one of the easiest furnaces

to build and run on a small scale (under 150 poundsof bronze).

Essentially you have a refractory lined cylinder whichhas a hole place perpendicular and near the bottom.

A low powered blower is inserted into the blower portor tueyer (tweer).

Then approximately 4 of charcoal is placed in thebottom and lit.

The crucible is placed on top of the warm coals and

charged loosely with metal. Charcoal is then placed around the crucible up to the

top lip. In some instances the crucible is actuallycovered with the charcoal (such as in melting copper).

The lid is then placed over the furnace and the blowerturned on

In adjusting the blower, you are looking for a neutralor slightly reducing flame. If there is smoke, you arereducing. If it is too oxidizing you are chancingoxidizing your metal and also may be burning toohigh. Adjust the blower to the point that it no longervisible effects the flame and then turn it down orshutter it slightly.

You may need to readjust the flame as the furnacegets going.

You may need to remove the lid and crucible duringlong melts and add more fuel.


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Oil drip furnace built by Colin Peck out of a beer keg. Thisparticular furnace runs on both waste engine oil as well as themore environmentally friendly, kitchen fryer oil. The furnace willhold around a #9 crucible and will get hot enough to melt iron as you can see he has cast his own crucibles out of iron fromflower pots below.

These images are by Colin Peck athttp://www.artfulbodgermetalcasting.com

Oil-fired Furnace An oil-fired furnace is

basically a cylindricalfurnace that uses any oilbased substance from waxto motor oil as a fuel.

The burner is designed sothat the oil drips into theburner tube and ignites.

They are very fast furnacesthat are inexpensive tooperate.

There is some smokeinvolved and the burnerscan be fussy they lack theease of gas furnaces.


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Various Oil Fired Furnaces

Kansas Brass Oil Fired Furnace


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Colin Pecks paraffin burner which uses a dryermotor and cookie tin in the construction.

There are a wide variety of designs for oil drip burners,but the basic principles remain the same. Drip oil

(pressurized or gravity fed) into a a burn chamber, whereit is ignited and the resulting flames pushed out througha burner tube. Some designs are fairly simple like theone below and others incredibly complex pressurizedsystems with filters for the oil.

Oil burner design


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Advantages and Disadvantages of oil Oil is readily available and easily stored. Although burning oil does produce

carbon, the source of oil is oftenrecycling either motor oil or cookingoil. (Note: used oil must be strainedbefore use and used motor oil cancontain toxins such as lead .)

If the burner goes out the oil will poolin the burner although this is lessdangerous than gas which can fill aroom and ignite.

It takes a larger burner to burn oil, it isharder to light and takes a few minutesto heat up.

Oil burners do not need any equipmentlike gauges that cannot be easily be

built.


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Oil fired furnaces can be run cupola style, witout acrucible - the metal is tapped out of the bottom. Inthis case, Colin Peck is melting copper. If diesel isused for fuel, you can actually melt iron.

Oil-fired furnace

Running the flame in slight reduction isstill important. This furnace is beingrun rich it has too much fuel in themixture.


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Gas Crucible Furnaces Professional gas fired

crucible furnaces,such as this one byMIFCO, will have ancast brick liner, anelectric start, a UVscanner (which assistsin starting and also

will automatically shutthe furnace down if itsenses a fluxation inthe flame).


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Round furnaces are more efficient thansquare furnaces.

Furnaces should be sized according tothe number crucible you are using.

They should be 4- 6 wider than thediameter of your crucible.

The furnace height, with liner, shouldaccount for a crucible block which getsthe crucible above the force of theflame and allow 1- 2 of space betweenthe lip of the crucible and the lid.

The burner port or tueyer (tweer)should be placed below the bottom ofthe crucible and be tangent to the sideof the furnace liner.

Depending on the metal you aremelting and the liner you are using,

allow for a liner thickness of 3- 6

There is a wide variation in furnace design, but as long as you stick to these basicprinciples, you should have a working furnace


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R2D2 looking furnace builtby Emre, a knifemakerfrom Turkey out of a LP

gas tank.


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Simple furnace that holds a#30 crucible. It actually hasa pair of tangentiallyoriented burner holes, but

only one is really necessary


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David Speers freon tank furnace

Gene Elliots coffee can furnace

Cowboy furnace by Lost &

Foundry Co. in Spokane, WA

Ideas are really only limited byyour imagination


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Burner Design There are also a wide variety of

burner designs First you need to determine

whether you are using naturalgas or propane (or another fuel

gas). BURNER DESIGNS AREDIFFERENT FOR EACH FUEL! You also need to decide

whether or not you are goingto use a blower or rely purelyon a venturi system. Generallyif your furnace uses over a #10crucible or will be meltingcopper based metals, you willneed a blower.

Propane venturi burners by Hybridburners.com


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Ron Reils burner designs are very popular among homebuilders looking for a venturi design.

Propane (LP) burners will always have a small interior hole that propels gas intothe burner tube surrounded by a series of exterior holes which allow the air to

mix into the gas (mixing chamber).


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Reduction through oxidizing on a

Venturi Burner

Image from Ron Reil at http://ronreil.abana.org


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Natural Gas Burners Natural Gas systems

should have someadded safety features:a vacuum switchwhich will turn off gasif the flame goes outand a UV scanner orheat probe to sensethe presence of flame.

An easily reachedemergency shut off

valve (or two) are alsoreccomended.


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Yet to come: design for the burner Iuse

Any one who wants to measure and draw out my burner and createa Solidworks or Rhino drawing will get 25 points of extra credit


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Safety Warnings about burners

Do not allow the flame to burn backinside the tube this can havedangerous results.

This is usually caused either by animproper mix ratio or a faulty design.

If you cannot get it to run without

burning inside of the tube, you needto reconsider your design. Frequentlythis is do to problems with yourair/gas mixture chamber.

Also you should consider a flash backarrestor on your line.

If you are running on natural gas it is

also recommended that you have aflame sensor of some type hooked upto a vacuum switch.


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Electric Resistance Crucible Furnace Basically electricity is run through

coiled wire that is slightly too small forthe amount of current that is beingforced through it. This resistancescauses heat as a by product.

Advantages: environmentally sound,produces only waste gas from themetal itself.

Disadvantages: melts slowly, mosthave a round a 2050 top temperature,

cannot melt in reduction, melts onlysmaller amounts.

All of the dangers that come withelectricity, plus additional ones if yourcrucible cracks inside the furnace.


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Homebuilt Electric Resistance Furnace

Dave Gingery has plans that can be found for building a furnace of thisnature and a few people offer suggestions for improvement, such as this

guy: http://www dansworkshop com

401 crucible furnaces

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