Kiln Maintenance For Metal Clay Artists

There are two types of kilns, brick and ceramic muffle. Mine is a muffle. Muffle kilns heat up faster than the brick type and can easily be moved from classroom to classroom without damaging the kiln. Its best to not move the brick type kiln because, movement can cause the bricks to become loose since they are held together with mortar.

Over time the muffle can develop cracks. I've been told that the cracks shown here in my kiln were caused by opening the door while the kiln is very hot. I'm guilty of this, due to opening the door while enameling. These types of cracks are not harmful. As long as the muffle isn't breaking apart in pieces, and the elements are still secure inside the muffle. As the muffle heats up it expands and the cracks close.

You should occasionally inspect your kiln by checking the thermocouple in the back of the kiln making sure it sticks out from the back of the kiln about 1/2". It can sometimes inadvertently get pushed into the muffle. The thermocouple measures the kiln's temperature and sends this information to the kiln's computer. If it is pushed into the muffle it can't accurately measure the temperature inside the kiln. 

If you are having problems with melting your metal clay while firing, then you should test the thermocouple for accuracy with a kiln tester. Here is a link to my instructions on testing the thermocouple: Testing Kiln.  

Sometimes the door's latch needs adjusting. The latch should lightly catch so that it doesn't shake the kiln. The door doesn't need to close tightly against the front of the kiln. There needs to be some space for the door's material to expand when it heats. If you work with enamels, you don't want the door to shake the kiln when closing otherwise you may find your enamel has fallen off your piece! On my kiln the door latch adjusts by twisting a screw in or out on the latch.  

It’s also good to check the kiln's plug wires making sure they are not cracked or damaged.  If they are seek advice from a kiln repair company.

What is the Reaction of Metal Clay with Aluminum?

Over the years I’ve been told not to place my moist metal clay on aluminum or there would be a reaction, so I never have. With that being said, I don’t know what the reaction is! Inquiring minds want to know now! I tested PMC3 and the new Sterling Silver metal clay to find out. 

I placed a sample of each clay on aluminum foil and also samples on an aluminum pan. I cut out a total of four oval discs labeling the discs that are laid on top of foil with an F and those laid on top of the pan with a P. I allowed them to sit there for an hour, until they were fairly dry. 

PMC3 & PMC Sterling on pan

PMC3 & PMC Sterling on foil

The two samples on the foil had a huge reaction as did the foil. 

Reaction to foil

Foil's reaction

The two samples on the pan had less reaction. 

Reaction on pan

Fired and polished sterling silver

It looks like the combination of moisture, clay, and aluminum causes the reaction. The PMC3 clay on the pan had less moisture and so had little to no reaction whereas, the foil seemed to trap the moisture between the clay and foil. I fired each sample per the manufacturer’s directions on the corresponding packages to see if the reaction area would burn clean. It did not. 

Fired and polished PMC3

After polishing each sample, the reaction areas are raised and extend past the surface of the clay. It gives the clay a look of reticulated metal. So, if you want a cool looking texture, now you know how to get it. Otherwise, keep the clay away from aluminum.

Part 1 - Testing of Sterling Silver Metal Clay in 2011

In September 2011 I was given Sterling Silver metal clay by Mitsubishi to test. I am now going to start posting these tests for you all now  that it isn't confidential information.

Test 1

• Simple Firing SS Clay in my kiln 
• Carving the clay 
• Doming after firing 
• Will SS Clay accept Aura 22 gold.

Made clay disc 2mm thick, 20mm wide. Imprinted it with a rubber stamp, and then carved into the center area. The clay chipped easily when carving lines close to each other.







Fired clay per instructions:
On Kiln shelf for 5 minutes at 840˚ F in Evenheat PMC kiln. Fired it in charcoal at 1500˚f holding it for 30 minutes.

Removed and placed into stainless steel container with provided charcoal.  Container sat in kiln 24mm up off the kiln floor.

Allowed it to cool for a 5 minutes, and then removed pan from oven using gloves. I opened pan, and using copper tongs I quenched the disc in a bowl of water. 
Disc sintered perfectly and kept all textures and carving detail.
It shrank from 20mm in diameter to 16 1/2mm diameter.

I then tested its malleability by doming with a steel dapping block.





There was no cracking. It formed perfectly.

I then polished it with a 3M red Radial Bristle Disc at 220 Grit.
I wanted to see if it would accept Aura 22 gold.
I painted three coats of PMC3 slip on the flower areas, allowing each coating to dry completely.  I then applied 3 coats of Aura 22 gold, allowing each layer to dry completely.
I then torch fired the disc until it was a peach color. Allowed it to air cool.
I then burnished the gold and polished the silver areas with the same 3M disc.

The gold sticks in some areas and flakes off in others.



Next week I will publish another test I completed in this series.

Testing Soldering Fired Sterling Siver Metal Clay

In this test I focused on soldering. I explored the following questions over a series of tests:

1. Does the porosity of the clay cause problems with soldering?
2. Do I need to burnish the clay first before soldering?
3. How well does hard, medium, and easy silver solder work with fired    clay?
4. Does pre-melting solder to the piece change the metal’s malleability? 
5. After soldering, can I form the metal without the sterling piece coming off or cracking?

I’d like to explain more about question 4, which relates to the changes of the metal. Melting solder onto the metal actually causes the solder to alloy with the metal (this is true with any non ferrous metal). The two metals combine at the molecular level to create a new metal with a different melting point. So, with this question I wanted to know whether the malleability of the new alloy differs from the PMC Sterling.

Test 1

In my first test, I made a two-tone ring with copper and metal clay. The center of the ring is made with fired PMC Sterling silver metal clay and the ring band is made from copper sheet.

 In my first experiment, I created a strip of textured clay 4 cards thick and fired it per the manufactures instructions. I only fired the clay for 30 minutes in the second stage firing at 1500˚F. 

I then pre-melted hard silver solder to the back of the sterling silver piece.

Afterwards, I sweat soldered* the sterling silver piece to the copper.

Once I started bending the copper/sterling silver into a ring, the sterling silver started cracking.

Here are my initials answers to my questions:

1. Does the porosity of the clay cause problems with soldering? No it does not.
2. Do I need to burnish the clay first before soldering? No. 
3. How well does hard, medium, and easy silver solder work with fired clay? Hard solder worked fine. 
4. Does pre-melting solder to the piece change the metal’s malleability?  It might, I will perform another test. More on that below.
5.  After soldering, can I form the metal without the sterling piece coming off or cracking? It doesn't come off but too much bend causes it to crack.

After soldering, can I form the metal without the sterling piece coming off or cracking? In this test, the metal cracked. More below.

*Sweat soldering is the process of melting solder to the back of one piece and then attaching it to another piece by again melting the solder, thus connecting both pieces together.

With the questions of malleability and bending still unanswered, I devised two more tests. 

Test 2

 

I created three PMC Sterling Silver metal clay bands using the same texture but made them 6 cards thick and fired them in the second stage for 2 hours at 1500˚F. I created three variations and bent them into rings with the following results:

• Band 1- No soldering/plain PMC Sterling band bent into a ring without any problems. 
• Band 2- I melted hard silver solder onto the band and then bent it into a ring without any problems. 
• Band 3- I repeated my experiment from Test 1, but wanted to see if making the band thicker and firing it for a longer time would keep it from cracking. I annealed the metal after bending it a little and bent it into an oval ring with few problems. It was only on the final, sharp bend that I experienced any cracks - just two small cracks that I was able to solder with medium silver solder. I also soldered the join successfully using medium silver solder.

Test 3

 I created a two-tone ring top using PMC Sterling and copper. I  domed it, and then soldered a ring band to it using easy silver solder. I used the plain PMC Sterling Silver ring band from Test 2 (Band 1).

The results: Everything soldered fine and the ring top domed without the sterling silver cracking.

My Conclusions

All silver solder types, hard, medium, and easy worked well on the metal with no burnishing required. I'm sure IT silver solder would work fine also.

Overall the sterling silver metal clay held up suitably while bending it into a ring band by itself. Doming and rounded bending of the fired metal clay also worked well. The sharp bending of the sterling attached to the copper was too much stress for it, but was easily fixed by making it thicker, firing it for a longer period of time in the carbon, and by annealing it during the bending process. Additionally, I soldered small cracks before they worsened. Sometimes even sterling silver textured wire can crack under these conditions. All in all, the fired sterling metal clay performed competently.