Carbon arc melting

Picture of Carbon Arc on marble

Making [[en:CaC2:::http://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Cac2.jpg/200px-Cac2.jpg::: carbide]] by arcing CaO and charcoal

Videoclip of arc melting cast iron

Introduction

Units are expressed in metric and prices in US dollars, unless otherwise stated.

An electric arc is a plasma of ionized gas which conducts electricity. Some industrial processes are based upon this like some steel melting and, the most widely used application is arc welding. In the past it was used for lighting as well, particularly for searchlights and cinemas. In this experiment heat transfer takes place mainly by blowing the plasma (like a torch) on the metal to be melted. You actually see a 'flame'. The color of the flame is heavily dependant on the medium which is heated. It matches the spectrum of the medium. When melting iron (or welding) it is blue, when heating lime it is pink (color of calcium), aluminum a bit bluish. As virtually every substance vaporizes and gets volatile, the spectral flame color every substance is visible, unlike an ordinary propane flame (max. 1400^oC) or even oxyfuel flame (max 2500^oC). As stated above it melts (mp 2600^oC) and vaporizes lime (CaO) easily with a pink arc color. It can be very interesting to view the arc with a specroscope like I did. Overview of features:

(+) Very high temperature can be reached quickly. However, the amounts to be melted are tiny (about 20 grams of iron at 150 A), but, compared to even an oxyfuel torch, the energy efficiency is tremendous. A torch takes a lot of longer time to melt 20 grams of iron which is melted by the arc within one minute.
(-) Temperature is hard to control. Much losses due to vaporizing may occur. Arc is disrupted easily. There should be enough vapor (from metal or refractory) to sustain the arc. A DC welder may do better as there is no cycling of 100 or 120 times per second. Platinum class metals (including iron and nickel) easily dissolve some carbon from the arcs which makes it brittle.

Important ! Additional safety

[[en:Arc_eye ]] Never look into the arc with(out) anything but a face shield designed for electric arc welding with the dark green glass. Failing to do this results in sitting upright in your bed with painful eyes the night after and, worse, permanent damage to your eyes may result ! Gas-welding goggles or ND4 gray filters are NOT suitable as they don't protect from the strong ultraviolet rays !!
When viewing with a spectroscope, do it as short as possible and shield off your other eye and face !
Always wear heat-resistant gloves and shoes easily to kick off.
Keep any inflammable stuff away from the arc and beyond reach of any sparks. Particularly melting even only 10 grams of iron or steel may spark as far as 1 m away. For this reason use protective clothing which is hardly inflammable.
The power switch (which also switches off the transformer or welder itself, not only the arc circuit) should be within reach in case of emergency.
A bucket of cold water or, better a running water tap to immerse burnt body parts in case of.
Ventilation hood. Because of the very high temperatures, most substances vaporize and/or decompose and may expel noxious fumes. These may consist of very small particles of silica or CaO which is condensed vapor. Do not inhale any fumes coming from the arc. Perform the experiments outdoors if possible.
Virtually any material (even the highest grade refractory, including the "unmeltable" magnesia) melts or even vaporizes in the arc. Take this in account.
Do not make arcs directly on the 230 V or 115 V mains as there is a strong and lethal electrical shock hazard. Moreover, the current is so low compared to a welder that only minimal amounts (< 1 g of iron) can be melted. The voltage between the electrodes or one of them and ground should not exceed 42 V for the same reason. Electric welders usually comply with this rule. In most countries (e.g. Western Europe) sale of unsafe welders is prohibited.

In short, generally safety rules for arc welding apply.

The welder Setup with charcoal and marble chips

Needed

A transformer yielding low voltage and high current (at least 100 A), preferably yielding DC does a good job. Electric welders, even the cheap hardware store AC buzzboxes do well. They are usually available for under $100 complete with welding mask, electrode holder (handpiece) and clamp.

Carbon rods. One option is using flat 4.5 V batteries with each three cells of which each cell has a carbon rod of 6mm (1/4") diameter * 80 mm. These batteries cost about $2-$3 each. But the remaining black slurry should be treated as hazardous waste and can leave unwashable stains as it is manganese dioxide (MnO₂).
A better option is graphite stirring rods of 6-12 mm diameter (1/4"-1/2") used for foundrywork, available in jewelry casting supply shops.
Take enough rods. Rods are consumed too due to combustion and vaporization. One 4.5 battery rod is consumed completely after 15-30 minutes of use, spread over more 'sessions'.
Charcoal chips may work as well, but they tend to break very easily.

Thick copper wire of at least 3 mm diameter (6 mm²), e.g. grounding wire. Remove the (usually yellow/green) insulation if present. Anneal before use to enable easy bending.

High-grade refractory which insulates well. Magnesia based refractories are best but alumina works well. Chamotte absorbs much heat. The gypsum based building blocks for inner walls from YTONG, designed for insulating inner walls of buildings, do well (see picture left) because of their excellent insulation properties. It will melt at about 1400^oC, so some slag will be formed around the arc area.
When not available, even ordinary bricks or concrete tiles can be used, as the extreme heat only occurs locally. Sizes of about 5x5x4 cm suffice. Carve a cavity of about 2-3 cm (1") diameter and 1 cm deep in it. This is the "crucible".

Extra clamp. The clamp of the welder might otherwise be damaged (the carbon rod gets so hot that where the carbon is in contact with the iron of the clamp, the iron melts too) so that it cannot be used anymore for welding, where the welder is actually designed for.

A hands-free welding helmet is more useful than an ordinary handheld mask. When holding the welder's handpiece with one electrode, the other hand is free to feed the metal into the puddle. The instability of the arc requires hand holding (and moving) of one electrode, so one hand is 'occupied'.

Schematic overview of the setup

Cut about 25 cm from the wire and attach the wire to one carbon rod by e.g. winding it one or two times around the rod. Make a tight connection without any play, otherwise it cannot bear currents of 100 A and more. The other end can just be clamped in the handpiece just like a welding rod. Clamp the other carbon rod in the free extra clamp and clamp that one in the clamp connected to the welder, close to the carbon rod to minimize resistance. Put the end of the latter rod above the center of the crucible. Fix it properly, as the heavy cable to the clamp tends to move the rod away (and fall on inflammable material). Start with a low current (50 A). First test it 'dry' (without metal). Hold the handpiece and let both electrodes touch. You see it sparking and making a sputtering noise. Pull the electrode in the handpiece slowly apart from the other one. The best arc is got when it is long (1 cm), shows a 'flame' and makes a humming sound rather than a loud sputtering sound. When it disrupts, make contact again and pull them apart again. After a few seconds the refractory melts. Even [[en:MgO:::http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Magnesium_oxide.jpg/200px-Magnesium_oxide.jpg::: magnesia]] or [[en:CaO pure lime (CaO)]] show a glossy appearance. You don't see much molten CaO or MgO, not because the temperature is not high enough, but because the boiling point is only 100-200^oC above the melting point, so it's rather volatile at the melting point. Now stop the arc and put some nails in the crucible. Start over again. After the arc starts, any iron close to the arc melts. Moving the arc over the nails or pushing the nails to the arc with an old screwdriver may speed up melting. The molten iron tends to contact with the carbon electrode(s), but try to prevent that. Avoid any direct contact of the carbon electrodes with the molten metal if possible. Otherwise the current flows through the iron (and the welder hums loudly, which also happens when an electrode 'sticks' during welding) and the arc is gone. Moreover the iron gets contaminated with the carbon and gets cast iron. After one minute you'll see that everything is molten. When is does not melt all (it freezes further from the arc) increase the current on the welder.

An excellent application is steel casting. As pispot steel (ordinary iron with less than 0.3 % carbon) always takes a little carbon from the rods, it is converted into steel after one melt. This is simple to check. When sawing it with a hacksaw and the saw gets blunt, it's clear it's steel.

Arc melting lime with 8mm rods

Arc melting iron, viewed through welding mask

Videoclip of making CaC₂
(flash player 6 req'd)

Here is a table which tells the behavior of different substances.

Behavior of different substances

Material Arc color Sustainability
of arc What happens ?

Iron or steel Blue - - Iron melts and tends to stick to electrodes when no slag added. Sparks well on the air.

Aluminum Light blue o Melts, boils and burns and leaves molten alumina (Al₂O₃)

Marble CaCO₃ Pink + CaCO₃ decomposes and marble crumbles, later the CaO melts (at 2600 C)

Marble CaCO₃ chips + charcoal Pink + CaCO₃ decomposes and marble crumbles, later the CaO reacts with the carbon which results in CaC₂ (liquid: freezes at 2300^oC). After cooling, this carbide reacts with water,releasing C₂H₂ acetylene gas.

Impure silica stones White o SiO₂ melts to a syrupy and sticky substance.

Crushed silica stones + powdered charcoal White o Reacts and small balls (< 1 mm) of liquid silicon are formed

Glass Pinkish orange ¹ - Breaks apart. Do not try without proper protection, as the glass breaks violently due to very poor heat conductivity !

- - = bad, - = poor, o = fair, + = good, ++ = excellent
¹ Depends on composition. Here, Na₂O and CaO containing glasses are considered.

Behavior of different substances
Material	Arc color	Sustainability of arc	What happens ?
Iron or steel	Blue	- -	Iron melts and tends to stick to electrodes when no slag added. Sparks well on the air.
Aluminum	Light blue	o	Melts, boils and burns and leaves molten alumina (Al₂O₃)
Marble CaCO₃	Pink	+	CaCO₃ decomposes and marble crumbles, later the CaO melts (at 2600 C)
Marble CaCO₃ chips + charcoal	Pink	+	CaCO₃ decomposes and marble crumbles, later the CaO reacts with the carbon which results in CaC₂ (liquid: freezes at 2300^oC). After cooling, this carbide reacts with water,releasing C₂H₂ acetylene gas.
Impure silica stones	White	o	SiO₂ melts to a syrupy and sticky substance.
Crushed silica stones + powdered charcoal	White	o	Reacts and small balls (< 1 mm) of liquid silicon are formed
Glass	Pinkish orange ¹	-	Breaks apart. Do not try without proper protection, as the glass breaks violently due to very poor heat conductivity !

For more info you can mail me.

More info on Wikipedia

Last update: Sat 8 Mar 2003

	A transformer yielding low voltage and high current (at least 100 A), preferably yielding DC does a good job. Electric welders, even the cheap hardware store AC buzzboxes do well. They are usually available for under $100 complete with welding mask, electrode holder (handpiece) and clamp.
	Carbon rods. One option is using flat 4.5 V batteries with each three cells of which each cell has a carbon rod of 6mm (1/4") diameter * 80 mm. These batteries cost about $2-$3 each. But the remaining black slurry should be treated as hazardous waste and can leave unwashable stains as it is manganese dioxide (MnO₂). A better option is graphite stirring rods of 6-12 mm diameter (1/4"-1/2") used for foundrywork, available in jewelry casting supply shops. Take enough rods. Rods are consumed too due to combustion and vaporization. One 4.5 battery rod is consumed completely after 15-30 minutes of use, spread over more 'sessions'. Charcoal chips may work as well, but they tend to break very easily.
	Thick copper wire of at least 3 mm diameter (6 mm²), e.g. grounding wire. Remove the (usually yellow/green) insulation if present. Anneal before use to enable easy bending.
	High-grade refractory which insulates well. Magnesia based refractories are best but alumina works well. Chamotte absorbs much heat. The gypsum based building blocks for inner walls from YTONG, designed for insulating inner walls of buildings, do well (see picture left) because of their excellent insulation properties. It will melt at about 1400^oC, so some slag will be formed around the arc area. When not available, even ordinary bricks or concrete tiles can be used, as the extreme heat only occurs locally. Sizes of about 5x5x4 cm suffice. Carve a cavity of about 2-3 cm (1") diameter and 1 cm deep in it. This is the "crucible".
	Extra clamp. The clamp of the welder might otherwise be damaged (the carbon rod gets so hot that where the carbon is in contact with the iron of the clamp, the iron melts too) so that it cannot be used anymore for welding, where the welder is actually designed for.
	A hands-free welding helmet is more useful than an ordinary handheld mask. When holding the welder's handpiece with one electrode, the other hand is free to feed the metal into the puddle. The instability of the arc requires hand holding (and moving) of one electrode, so one hand is 'occupied'.