What is Activated Carbon and how does it work?

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.

How to apply 24k gold Kumboo on Metal Clay or Fine Silver

Keumboo on Metal Clay or Fine Silver
Keum-boo is the art of bonding pure gold foil over fine silver.  As the fine silver and 24k gold molecules heat up on a hotplate, the two metals are joined through the process of burnishing the metals together. The two metals bond by sharing oxygen molecules.

Materials

• Finished fine silver item (PMC or fine silver)
• 24k gold, 23 ½k, or 23k (can be purchased from Allcraft tools (212) 279-7077)
• Hot plate - with high, medium and low settings or a Ultra Lite Kiln with a coil cover
• Klyr Fire by Thompson Enamels or thinned Elmer’s Glue
• Small craft paint brush
• Burnishing tool
• Fine tweezers
• Heat resistant gloves (lightweight leather gloves)
• Long tweezers (12")
• Heat safe work area  or fiber board
• Small cup of water
• Paper towels

Note

Gold leaf is less expensive than gold foil because it is much thinner. This thinness also makes leaf more difficult to handle. It also requires the application of several layers of leaf to obtain the same appearance as foil.Gold foil is available through the PMC Connection at www.pmcc.com or by phone to Allcraft tools.

The Process 

Precondition: The silver item (PMC) piece is already, fired. Do not brass brush, burnish, or tumble the PMC. All areas that will have gold applied must be burnished.

• Align all tools so that they are easily accessible, 
• If needed, place a metal cover over the hotplate so as  to give it a smooth even heating surface. I use a sheet of think copper. 
• Preheat the hotplate to high.
•  Its up to temperature when a toothpick burns when placed against the burner.
• Apply watered down glue to the first area to have gold applied to it. 
• Place the gold foil inside a folded sheet of paper. 
• Cut the shape out with the foil inside the paper. 
• Remove the gold from the protective paper using tweezers and place it on the wet glue.  You can also pickup the gold with a wet craft brush and place it on the metal by painting the foil down against the metal.
• Smooth the foil against the metal using a craft brush. 
• Place the metal piece on the hotplate using tweezers. Allow it to heat up to temperature. 
• Hold the piece with long tweezers, gently dab at the gold with a burnisher.
• You will know its up to temperature (650˚) when the gold starts sticking down onto the metal.
• Apply even strokes across the gold so that every millimeter is burnished to the silver. 

Notice that the gold is shiny where it is burnished to the silver. If the burnisher becomes too warm cool it by dipping into water and then drying it with a towel. 

• Repeat the process of applying the gold to the silver as wanted. 
• Finish the piece by burnishing with a soft brass brush with soapy water. 

 Notice: Re-firing the PMC in the kiln or with a torch over 1110˚ allows the gold to alloy with the silver causing the gold to disappear!

Testing your Kiln

Testing Your Kiln’s  Temperature By Janet Alexander

Why test?

Now that most kilns have a computerized controller it’s become easier
to control the kiln’s temperature. But, is the controller’s readout accurate?  With
the varieties of bronze, copper, and silver metal clays it has become
important to know if your kiln’s temperature’s reading is correct.  Otherwise
you may have problems with the clay not sintering correctly or getting
too hot and melting.  Additionally, the kiln should be tested
for hot spots and cooler spots.  Every kiln is different.

Testing can be done by using a pyrometer (which is how I used to adjust
my kiln’s temperature before there were controllers) or by using kiln
pyrometric cones. The pyrometric cones are supposed to bend when heated
to a specific temperature.  Pyrometric cones give a temperature
equivalent; they are not simple temperature-measuring devices. According
to a pyrometric cone manufacturer, these cones have over 20 variables
that can affect the cone’s bending. Some of these variables include:  cone
composition, particle size of raw materials, type of forming process,
moisture during forming, density of the part, geometry of the part, setting
height and angle and the heating rate. Atmosphere also affects bending
behavior. Wow, that’s a lot of variables!  So, let’s look at
testing with a pyrometer or a kiln test kit.

I used the kiln test kit sold by the PMC Connection. According to experts,
a controller is accurate to ±10°F (±5.5°C) so keep
this in mind while testing.  Additionally, run the test several times
but move the thermocouple to different areas of your kiln. Place it towards
the back, near the front, off to each side, and etc.  Test at
different temperatures. I conducted tests at 1110˚F,
1200˚F, 1290˚F, 1470˚F, 1560˚F, and 1650˚F. I found that both readouts were within a few degrees
of each other until the temperature got up to 1650˚F.
Then they were off by 10˚F! So, I added another
thermocouple from my casting kiln!  All three read different
degrees but were within the accuracy range.  Now I know to lower
the temperature of my kiln by a few degrees when setting it at 1650˚F.  Now I know why my PMC3 clay has a crystalline
look to it when I fired it at 1650˚F; it was
getting too hot!

Instructions

The test kit includes: Sensor reader (tester) 9V Battery K-type thermocouple TP-02A K-type thermocouple TP-03 Case Instructions (not well written) Use the TP-02A thermocouple (larger one) for testing your kiln. It has a temperature measuring range of (-58˚F to 1650˚F).
Install the 9 V battery into the unit.
Insert the plug into the bottom of the sensor reader making sure the plug’s polarity matches with the sensor’s polarity.
Insert the thermocouple into the kiln.
Caution: Don’t insert it past larger ceramic end or the wires will burn!
Place the thermocouple near the kiln’s thermocouple.
Turn on kiln and set it to hold at the test temperature for at least 15 minutes. The sensor reader can read in Fahrenheit (F) or Centigrade (C). Turn on the sensor reader by sliding the button from the center (off position) to the F (if measuring in Fahrenheit).
The sensor displays its reading. Allow a minute to stabilize to the temperature.
When finished testing, turn off kiln and sensor reader and allow the thermocouple to cool before touching it.