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Monday, October 17, 2011

What is Activated Carbon and how does it work?

What is Activated Carbon








By Janet Alexander


There are actually over 150 types of granular, powdered, and pelleted activated carbons. The different carbon types are created for different purposes. Some are made to absorb organic chemicals, pollutions, or filter liquids. For example, carbons with large holes are used for filtering fish tank water, while carbons with small holes are used in air filters. We use it to capture fumes while sintering metal clay. Not all activated carbons are of the same quality. This is why it’s important to buy activated carbon from a supplier who has completed extensive tests on the carbon they sell.

At first glance this photo may look like a thunder storm or smoke from a fire, but it is actually a micro photo of steam activated carbon made from coconut shell. I was asked to write about the carbon we use in the process of sintering metal clay, so I set out to find out more information on activated carbon. I’ve heard it called acid carbon, coal carbon, activated carbon, acid washed carbon, steam washed carbon, rainbow carbon, and coconut carbon etc. So, what is it anyway?

What is activated carbon?

Activated carbon can be manufactured from any organic material. Commercial carbons are made from sawdust, wood, charcoal, peat, lignite, petroleum coke, bituminous coal, and coconut shells. We use activated coal carbon and activated coconut carbon in the sintering process for metal clay. Activated carbon is a carbon which is chemically treated, or steamed to enhance its absorbing properties.

Coal Activation
According to Calgon Carbon, a manufacturer of activated carbon, “the coal is pulverized to a very fine particle, about the size of talcum powder. The powdered coal is mixed with a binder to "glue" it back together and pressed into briquettes. These in turn are crushed and classified to the size of the desired end product.” 1
The coal is heated in an oxygen free oven to remove the unstable components of the coal. The carbon is then activated by heating it again in an oxygen and steam environment. The activation process creates a highly porous coal with remarkable surface area. 1

Chemical Activation
Wood type products are activated using chemical activation. The material is mixed with activating and dehydrating chemicals (acids) and then heated between 932 - 1472˚F. The acid causes the wood to swell, opening the cellulose structure and stabilizes this structure, keeping it open. The acid is then washed out of the carbon.2
 
Steamed Activation
Peat, coal, coconut shells, lignite, anthracite, and wood are activated using steam activation. The material is converted to carbon through heating. Then it is cut into 0.35nm thick chips (looks like potato chips). They are placed in a jumbled pile and are heated to 1835˚F and at the same time they are blasted with steam heated to 266˚F. The steam creates pores in the carbon. Depending on the original material used the pores are very small or can be large. The pores in hard coconut shell carbon are very small, micro pores. The pores formed in peat are usually meso sized pores.2

The performance of a carbon is based upon the types and number of internal pore sizes, the internal surface area, and percent of ash in the carbon. The most important determining factor for carbon use and performance is pore structure.
There are three sizes of pore measurements.
  • Micropores have a radius of less than 1 nanometer* (nm) and are the smallest of openings in the carbon or less than 40 angstroms.**
  • Mesopores have a radius of 1 -25 nm.
  • Macropores have a radius of larger than 25nm or above 5,000 angstroms.
Since the carbon we use is pulverized, it only has micro and meso pores.Carbon is pulverized into various mesh sizes. On her blog site, Hadar Jacobson3 refers to using a size 12 x 40 coconut shell-based carbon, such as what the PMC Connection sells. Additionally, she states, “…we want carbon that does not produce a lot of ash and does not stay hot a long time after firing.”

What do these pores do?

Now that the carbon has been activated (made more porous) what does that have to do with how it works?
The smaller pores that are very close together create an energy field. This energy field, on a molecule level, attaches to the contaminant and adsorbs the contaminant. It is a chemical attraction.1

The more surface area in the activated carbon,the more it can absorb contaminants. In essence, the activated carbon (both coconut and coal) work the same. After reviewing the processes of activation, we now know that the activated coconut carbon has many more total pores and micro pores than the activated coal carbon. So, the activated coconut carbon may be more efficient at absorbing fumes than the activated coal carbon. Their only differences are their PH levels, ash content, and the volume they can absorb. The ash content reduces the overall activity of the carbon and the efficiency of reactivation.  The PH level is only important when using activated carbon for filtering liquids, as the PH level of the activated carbon can change the PH level of the filtered liquid. The hardness is also important.  The harder the activated carbon the less it crushes making more dust.

During  the metal clay sintering process, the activated carbon traps oxygen from inside the container it’s placed in and free radicals from the metal sintering.4 Sintering is basically heating the metal clay particles so that they fuse together. The activated carbon keeps the tiny metal particles in the metal clay from oxidizing while they are heated.  If they oxidize too much they can’t join together.So, what makes the coal activated carbon make the rainbow colors on the bonze clay? I haven’t found an answer yet. Why is it better to us coconut activated carbon when sintering copper clay and PMC Pro clay? My theory is that the coconut activated carbon is able to absorb faster than the activated coal carbon since it has more total pores to work with.

What is spent carbon and reactivation?

Over time the carbon pores fill up with the contaminant (now called adsorbate) and its absorbing power is gone. The carbon is “spent,” and no longer works.  Reactivation is a process of cleaning the pores so that the carbon can work again.There are three processes used for reactivation.
  • Use heat (thermal recycling).The heat vaporizes or burns off the adsorbate inside the pores. The carbon is reactivated between
    1292 - 1832˚F. 2 
  • Use steam (steam recycling). Steam is hard for the amateur to process at home.
  • Using boiling water. 2
To reactivate the carbon using heat, place it inside the stainless steel container, cover with the lid, and fire for 30 minutes at 1750°F. Allow it to cool in the oven with the lid on. Then sift out the ash by pouring it from pan to pan while blowing on it lightly, or take it outside with a light breeze blowing and pour it from pan to pan.

Warning! It can catch fire in an oxygen environment at 392˚F and above! It is best to keep it covered to avoid a fire.
To reactivate the carbon with boiling water, place it into a sauce pan of boiling water (ratio 2 to 1), stir with a spoon. Soak the carbon until it sinks to the bottom of the pan and then pour off excess water. Repeat 4 - 5 times. Place the carbon again into boiling water and allow soaking it for 24 hours.2 Dry the carbon by placing it on flat tray in the kitchen oven or toaster oven and heat at a low temperature (150˚F) until dry.

I’ve been told to keep the activated carbon in an air tight container. Several artists have stated to me that they
don’t do this and haven’t had any problems.  However, if you think about it, the carbon is made to absorb airborne particles and fumes, so by keeping it in an air tight container, it cannot become spent by simply sitting in open air. 

General Instructions for firing metal clay in activated carbon

  • Always test your kiln’s temperature accuracy and adjust the kiln’s temperature
    accordingly. The PMC Connection sells a testing unit.  
  • It’s always best to test fire samples before actually firing your creations.
  • Find the cool and hot spots in your kiln by using testing sample or using a
    temperature tester.
  • Place same-size pieces in the stainless steel container. If firing smaller pieces
    with larger pieces, place the smaller pieces in the cooler area to compensate
    for their size.
  • To evenly heat the container, elevate it approximately 1” above the kiln
    floor by sitting it on top of fire bricks or kiln feet and place it in
    the center of the kiln.
  • Place at least 1” of activated carbon under your pieces and ½” to 2” above them.
  • Keep the peaces at least ½ “apart.
  • Follow the manufacturer’s instructions for firing the clay.
  • Allow 1” of air above the activated carbon if using a lid on the container.

References


1  http://www.calgoncarbon.com/carbon_products/faqs.html
2Gert Strand, “Activated Carbon for Purification of Alcohol.”
3 Hadar Jacobson, “Instruction manual for Hadar’s Clay
http://artinsilver.com/Quick-fire_clay_instruction_manual.pdf
4 Gary Busby, chemical engineer.
* nanometer:  One nanometer is one billionth of metre(1/1000000000 of a metre, or 0.000000001 m). It is often used to express dimensions on the atomic scale.   
**
angstroms:  A unit of length equal to 1/10000000000 (one ten billionth) of a meter.

9 comments:

  1. I had Dave S. email me about his thoughts on carbon. This is what he said.


    I am a dabbler in PMC, I don't have a kiln, but I do have a strong science background; however evaluate them and use them how you will.

    Much of your discussion on activated charcoal was taken from the literature on oderant adsorption. Oderants are reactive molecules--usually double bonds and other special configurations. None of this is relevant to its use in PMC work except for the fact that
    activation processes result in really clean charcoal.

    The reason for the charcoal is that at kiln temperature--or above 700F anyway--it will react with any oxygen around. The carbon will burn to CO2 or CO and remove the oxygen before the copper (in Sterling, or bronze as well as copper clay) gets hot enough to react with it. Tarnish and fire scale and all that nasty stuff is usually copper oxides.

    Now for some of the ifs ands and buts.The clay binder must be burned off in air. It, like most hydrocarbons will ignite in air in the 600 to
    800F range. Once ignited, it will burn at temperatures of 2000F or higher but there is so little of it that it is gone before any damage to the metal clay occurs. Now all that is needed is to sinter copper containing clays at temperatures where the metal softens but doesn't really melt--say between 1600 and 1700F in the absence of oxygen/air.
    Charcoal does remove the oxygen, but in doing so it burns at temperatures above 2000F which will ruin the metal clay object. Given sufficient air this will happen regardless of the kiln thermostat setting! This is the source of almost all the problems people talk about
    on the metal clay blog.

    So the closed kiln, the perforated containers, the blankets etc; plus the thickness of the surrounding charcoal serve to keep atmospheric
    oxygen from getting to the charcoal layer in actual contact with the metal clay object. The theory is that whatever oxygen leaks in is
    consumed in the outer layers of the charcoal. The charcoal itself serves to insulate the inner layers and the PMC object from the high
    temperatures produced by this combustion. This can and very often does work. But the actual temperature seen by the clay may be unpredictably above the set temperature. It has always surprised me that people don't blow nitrogen or another inert gas through their kilns.

    But there are hidden sources of oxygen (air) in the system! All the pores of the PMC are filled with it. Large/thick clay objects hold more than small, thin ones. That clay mandrill you used to control ring shrinkage is permeated by it. This oxygen will leak out slowly as the
    kiln heats up, maybe mostly from the largest pores. It may well create hot spots as it burns the charcoal in contact with the PMC object. Inert gas pretreatments might eliminate this problem.

    Another possible problem is that impurities in the charcoal may decompose/gasify at kiln temperatures and one way or another react to
    produce high temperatures, uncontrolled by the kiln thermostat. Thus treatments that clean the charcoal may help. Preheating it to working
    temperature for some time should really help.

    This is a bit complicated, but keeping these things in mind as you experiment might help you to find what works all of the time (or almost
    all the time anyway).

    Hope I didn't bore you.
    I really think most the firing problems are due to over temperature caused by burning charcoal.
    Keep up the good work.

    Dave Shephard

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  2. Wow great info, thanks so much Janet!!

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  3. Hello Janet,
    I'm Jérémie Ketels, the french Engineer who developed Meteor Metal clays and the M20 kiln. My English is very poor but I think that I can manage to be understood. For several years, everybody think that activated carbon is the best product to protect objects containing copper from oxygen. In reality, the best product is by far the simple and natural barbecue charcoal. The reason is that the natural barbecue is made with low density wood who burn much more early in the kiln (lower auto-ignition point). You just have to crush it before use. Activation has no effect for metal clay, it’s a waste of money and it’s not good for sintering.
    Debinding is a very important phase. Don’t use any carbon in this phase. Most of time, 2 or 3 minutes are very sufficient. Use a wire mesh to do this, above a blue gas flame. When the object became glowing red, stop the debinding. You can burn the binder in the kiln too, but most of time, the oxidation is much more important (too long debinding time).
    All debinding methods give oxidation on the objects. In the second phase in carbon, if we only prevent oxygen action, the objects will stay oxidized. We have to “reduce” the black copper oxide. Don’t use a lid and let the carbon react with oxygen to give CO monoxide gas. At high temperature, CO will react with the oxygen of the oxidized metal to form CO2.
    This is why blowing Nitrogen or inert gas in the second phase is absolutely not a good idea ! This can only be done when metal powder object are formed by compression, without any binder. No binder = no oxidation generated during debinding. A Oxygen-free mono-phase firing is only possible if no binder is used, but it need strong molds to obtain a good level of compaction.
    I wish you a good luck for your future firing-tests !

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  4. Activated carbon is a form of carbon obtained from charcoal that features a very large surface area available for chemical reactions and absorptions. Activated carbon is manufactured to very be porous and therefore able to achieve a great degree of absorption. Activated carbon is also known as "Active" carbon.

    Dafco Aerostar 20x20x1 MERV 6

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  5. My 2 cents: From my long experience as a potter using; oil, LP gas, natural gas, wood, hydrogen gas, pine and mangrove charcoal to reduce metallic oxides in glazes and how that works...I'll have to side with M. Ketels. I think all those pores in activated charcoal have no real advantage and it's hard for me to swallow the 'energy field' theory. Other than being messy I don't understand why more ash is undesirable or a negative. I believe Coal it's self is a 'dirtier' or less pure form of carbon containing oxides of silicon, aluminum, iron, calcium, magnesium, titanium, sodium, potassium, arsenic, mercury, and sulfur plus small quantities of uranium and thorium. Some of those things probably account for the rainbow effects. Has any one tryed soaking charcoal in salt or adding other materials for special effects?

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  6. My husband was a stoneware potter, doing a lot of reduced glazes. I am a bronzeclay and copperclay user . I tried activated and barbecue coal and I mus t say that Jeremy kettle is right

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  7. Activated charcoal powder is an amazing substance and there are many benefits of activated charcoal. It adsorbs more poisons than any other substance known to mankind. liquid charcoal

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  8. Very interesting thread and I am just now reading it! Nice job, Janet. I'd like to reply to Dave S.' comments about inert gases: Ed and I tried several times to no avail to fire base metal clays to sinter with several inert gases. The cost factor alone would prohibit most people from trying this. I would say that it would work in theory, and in practice too after lots of trials, but it just was not cost effective for us to continue testing. Thanks.

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