Day 4 of Metal Clay Sintering Tests

I’m probably getting closer to my ideal phase two firing temperature now, so I’m going with:

2-cards thick ¾ x 1½ inch rectangle

7-cards thick ¾ x 1½ inch rectangle (“7” was more mid-way than “8”)

12-cards thick ¾ x 1½ inch rectangle

Ramp 1                         500°F/hr

Holding Temp 1           1420 for 1 hour

Carbon Under               ½ inch

Carbon Over                 ½ inch

Ramp 2                         full

Holding Temp 2           1470 (was previously 1450) for 2 hours

I’m anxious to find my best phase two temp so I can start messing around with phase one methods and temps.

I supposed I should confirm that my first test pieces are fully sintered by doing the sanding test.  After all, I’m keeping the phase one temp at 1420 because of my assumption that the pieces are sintered.

I was in the middle of an online class when phase two finished, so I didn’t get to the kiln until it had already cooled to 500 degrees, but I don’t really think that made any kind of difference.

The 2-cards thick piece had some blistering.  The 7-cards thick piece had a lot more blistering.  The 12-cards thick piece had no blistering, but all three pieces showed pitting on the bottom (the surface facing downward in the carbon). 

 

The 12-cards thick piece has more pitting because it's heavier than the other pieces so more weight is pressing into the carbon than the lighter pieces. 

This is good.  We now know that 1450 works (for these particular bronze pieces) and 1470 is too high.  

For my (hopefully) final test of phase two firing schedule for Brilliant Bronze, I only made one 7-cards thick piece.  

The piece is currently in the kiln for phase one, then after cooling I’ll run phase two at 1460 to determine the final optimal phase two temperature for mid-fire schedule.

Day 3 of Metal Clay Sintering Testing

As mentioned previously... for my next test, I’m going to keep everything the same except the final holding temp.

Ramp 1                         500°F/hr

Holding Temp 1           1420 for 1 hour

Carbon Under              ½ inch

Carbon Over                ½ inch

Ramp 2                         full

Holding Temp 2           1450 (was previously 1420) for 2 hours

And I’m only doing…

6-cards thick ¾ x 1½ inch rectangle

8-cards thick ¾ x 1½ inch rectangle

12-cards thick ¾ x 1½ inch rectangle

I guess only two things can affect the sintering process.  One, the binder doesn’t burn out properly… or two, there isn’t enough heat to cause sintering.

So if I find out the highest temp I can go without showing signs of over-firing, then I can work on the other factor (making sure my binder burns out completely).

At the end of phase one, I cracked the lid until the temp dropped to 1000, at which point I opened the lid all the way for about 15 minutes.  Then I closed the lid and went to bed.

In the morning, I started phase two.  At the end of phase two, I cracked the lid until the temp dropped to 1000, then I removed my pieces.

Here’s what they look like straight out of the kiln.

I repeated the “sound” test.  All three pieces made metallic sounds.  This 12-cards thick piece made a more metallic sound than the 12-cards thick piece from test 1.

I repeated the “water” test and although the 12-cards thick piece did absorb the water drop, it took about 30 seconds.  As you recall from yesterday’s video of the test 1 pieces, the 12-cards thick water absorption was almost instantaneous.

Just to compare, I flipped the pieces over and did the water test on the backs (the sides that were face down in the kiln).  The water seemed to absorb faster on the backside than on the top side.

Now to bend and break them so I can see what’s inside and compare them to the inside of test 1.

I have mixed feelings about bending.  I want the metal to bend and not break (not sure why) and yet I need it to break so I can see what’s inside.

First I bent them into 90-degree angles.  Not a single crack.

Then I bent them into parallel U shapes.  6 and 8-cards thick were fine.  When I got a small crack on the 12-cards thick piece, I stopped bending it.

Next, I continued to bend the 12-cards thick piece so I could see what the inside looks like.

As you can see, there is no part or edge inside that is thicker (more compacted) than any other part.  That bit on the upper right side of the bottom piece is not a thicker edge as I first thought.  It is where the piece started curving because I bent it.  

The white areas on the upper part are where I ran my fingernail across the inside.  The fact that my fingernail could burnish it rather than finding powder, leads me to believe this is sintered.

Perhaps there is no way to make the interior less porous.

But since these pieces all came out with no signs of over-firing.  I think I’ll repeat the test with a higher phase two temperature so I can find the point that lies just below over-firing.  Although I still assert that it could be a different point for different thicknesses, shapes, and/or combinations of clay. 

Come back tomorrow to see the next results.

Day 2 of My Metal Clay Sintering and Kiln Tests

This is a continuation of my metal clay firing schedule tests.  See previous blog entries for more explanation.

After phase one, I didn’t notice the kiln was finished until the temp was already down to 900 degrees, at which point I just opened the lid fully.  About 20 minutes later, I closed the lid and left it until morning.  In the morning, I blew off some of the ash and added the thinnest layer of replacement carbon.  I then set it to a full ramp to 1420 with a hold time of two hours.

As soon as the hold time was finished, I cracked the kiln open and let it cool down to below 1000 degrees, then I opened it all the way and sifted my pieces out.

I did the “sound” test by dropping each piece onto my steel bench block.  They all made different sounds (see video).  I go in order, 2 then 4, 6, 8, and finally 12-cards thick.

Then I did the water test by putting a drop of slightly sudsy water onto each piece.  The 2 and 4-cards thick pieces easily repelled the water.  6 and 8-cards thick repelled the water but it spread out onto the piece rather than staying in one lump drop form as in the 2 and 4-cards thick pieces.  12-cards thick totally absorbed the water.  One side of the 20-cards thick cube absorbed the water, but some of the other sides seemed to repel it.  (See video)

The sound test, the water test, and the bend test (see next) could mean absolutely nothing important.  I am merely sharing with you some of my experiences.  You will read all kinds of different info about metal clay, testing, and sintering.  The thing is, there are also many KINDS of metal clay and they are not created equal.  So I think the most important thing to do is follow the manufacturer's recommendations.  I just happen to like experimenting and since I'm doing kiln testing anyway, I'm throwing in some added experiments.

Next, I took pliers to the pieces and tried to bend them into 90-degree angles. The 12-cards thick piece snapped rather than bending.  

Here’s a close-up of the interior of the 12-cards thick piece.

The cube I sawed in half, but I stopped right before I got to the bottom because I knew that the act of sawing the cube would burnish the interior sides thus making it impossible to tell if the inside was sintered.  As you can see, where I stopped sawing looks similar to the 12-cards thick interior.  Very porous or powdery even.

So my question is: Did the thicker pieces not fully sinter because of something during the binder-burning off stage or because they simple need higher or more heat in the second phase of the firing schedule?  Is it useful or even possible to get the interior of a thick piece of metal clay fully sintered?  Maybe this IS fully sintered.

And what factor to I alter for my next test?

I’m not saying I need to be able to bend metal clay after firing.  Or even that it’s expected to be able to bend.  I just think the more I experiment with what metal clay can and can’t do, I’m working work toward making the strongest product possible.

Next I decided to fold the pieces over completely in half.  The 2 and 4-cards thick pieces took it well.

The 6-cards thick piece cracked where there had already been a crack showing on its surface.  And the 8-cards thick piece cracked in half completely.

The interior of these broken pieces LOOKS powdery, but it’s not.  If I rub it (or burnish it), it shines like metal.  Rubbing vigorously does not dislodge any particles.

I want to compare pieces from different tests to see if the edge color can go any deeper into the interior of the piece.

I forced  the 6-cards thick piece to break in half then I hammered the halves from 6, 8, and 12-cards thick.  They all flattened without any cracking.

So for my next test, I’m going to keep everything the same except I'll raiser the final holding temp.