I've been having some problem with PMC pieces not fully sintering and breaking after firing. Generally my pieces are 4 cards thick, and sometimes it's the main item that breaks, other times it's when I attach two dry pieces with slip. They appear to fuse when they come out of the kiln and then break entirely or break apart later, after the final finishing. I'm not sure if this is because my kiln is not calibrated correctly or it's because on some pieces I added PMC extender liquid to the clay or had too much oil on my hands that has seeped into the clay. I'm firing according to the package recommendations. Does anyone know if firing at the highest temperature for the appropriate clay at the lowest time is better than firing at the lower temperature for a higher time makes for a stronger product? Is there any problem with increasing the amount of time over and above what is recommended for a certain temperature. I'm using mostly PMC3 but occasionally work in PMC+. Any help would be greatly appreciated.

What you're going through must be so frusterating! The minimum thickness for durability is considered to be 3 cards thick, so if you're using 4 cards (assuming that you don't use a high-rellief texture on the 4 card thickness) you should be fine. The PMC extender and the oil on your hands shouldn't affect the strength of the piece. Here are a couple of suggestions. I hope this will help.

1. I fire 95% of my pieces at 1650 for 2 hours. This makes the piece as strong as it can possibly be because it's the hottest and longest firing schedule. However, if you have sterling or glass inclusions in your piece this firing schedule won't work, so adjust it as necessary. There's no problem for increasing the amount of time that you fire for, but firing for 2 hours and 30 minutes is not going to make it any stronger than the 2 hour schedule.

2. When making additions, I always attach pieces in the same state. Green PMC to green PMC (green is a ceramic term, its when the clay is dry but before firing) or fired PMC to fired PMC (you'll have better luck if you don't tumble/burnish the pieces before attaching).

3. Different formulas of PMC can be mixed together in the same piece (PMC standard, PMC+, and PMC3); HOWEVER, make sure you fire the piece so that all formulas of PMC are fully sintered. For example, if I made a piece using PMC Standard and PMC3 I could not torch fire it, I would have to fire it at 1650 for 2 hours. I'm wondering if this might be what's going on with your pieces breaking ... maybe you're using several formulas of PMC in a piece and not allowing all of them to fully sinter.

4. If you really think that your kiln is the problem you can go to a ceramic supply store and get something to test the temperatures that your kiln is reaching (sorry, the name of this product totally escapes me at the moment).

5. Make sure you brass brush & tumble your piece. Use extreme caution when hammering fired PMC (I've broken several pieces this way, its just the nature of PMC). Make sure you don't use a spring-loaded center punch to make the divet for drilling holes in fired PMC because that may also shatter the piece.

Good luck! Please let us know what happens!
Mollie

Hi, Mollie. I'm making mostly lentil beads with no added materials. I started out using PMC+ and then went to PMC3 which I mostly use for everything now. It could be two things that I can think of, one that some of the PMC+ pieces had been joined with PMC3 slip, and two, there was a batch that I didn't tumble, I just polished them with Rio's Pro Polishing Pads. They are wonderful and bring up a great shine, but I think the tumbling must add to the hardness of the piece. I know what you mean about hammering finished pieces, that's how I've broken most of them.

As for firing temps, do you fire PMC3 at 1650 for 2 hours? According to the firing schedule on the enclosed pamphlet that comes with the clay, two hours was the time for the original PMC clay. I didn't realize I could fire 3 or + that long without having it melt. I remember in the first certified class I took that the instructor said if it got too hot it would just melt the entire piece. If that is the problem, I'll just fire my pieces for a longer amount of time.

Thanks again for your prompt reply.
Sue

Hi Sue,

Joining PMC+ with PMC3 slip shouldn't be a problem, I've done it before. Celie Fago has a seperate slip jar for each formula, but Tim McCreight and CeCe Wire have 1 slip pot with all formulas in it. I only have one slip pot too. If the lentils are breaking apart near the seam they 2 halves may not be joined properly (make sure you use a wet paint brush to make the joining edges moist, then add slip, then join). Tumbling with stainless steel shot makes a HUGE difference. I tumble my pieces for 30 minutes to an hour. And yes, I do fire my PMC3 pieces for 2 hours at 1650 (this is a tip I got from Tim McCreight and CeCe Wire, you'll get the best sintering at this firing schedule for any formula).

Good luck!
Mollie

Mollie has given you very good advice. Most of the best teacher in the country will tell you to fire for the max time at the max temp for best results.

Krystal

Hi Molly,

Your suggestion made a huge difference. The pieces did shrink more by firing for 2 hours at 1650 instead of the recommended time on the package. I found it difficult to hand polish the new pieces because they are so hard, but they shined up beautifully in my tumbler. In fact, they took less time to come to a high shine in this harder state. They are also so much denser that I may try just three cards next time instead of 4. Thanks so much for your suggestions, I feel much more confident about my pendant pieces now and I don't think breakage will ever be a problem again.

Sue

I'm so glad to hear that you're problem's solved! :) I'd love to see pics if you have a chance to post them.

Mollie

I am firing my PMC with the Speedfire Cone System, following the firing instructions included for PMCPlus and PMC3. Can I fire longer with the Speedfire Cone System; would you recommend it? Chris

QUOTE: Originally posted by MollieM What you're going through must be so frusterating! The minimum thickness for durability is considered to be 3 cards thick, so if you're using 4 cards (assuming that you don't use a high-rellief texture on the 4 card thickness) you should be fine. The PMC extender and the oil on your hands shouldn't affect the strength of the piece. Here are a couple of suggestions. I hope this will help. 1. I fire 95% of my pieces at 1650 for 2 hours. This makes the piece as strong as it can possibly be because it's the hottest and longest firing schedule. However, if you have sterling or glass inclusions in your piece this firing schedule won't work, so adjust it as necessary. There's no problem for increasing the amount of time that you fire for, but firing for 2 hours and 30 minutes is not going to make it any stronger than the 2 hour schedule. 2. When making additions, I always attach pieces in the same state. Green PMC to green PMC (green is a ceramic term, its when the clay is dry but before firing) or fired PMC to fired PMC (you'll have better luck if you don't tumble/burnish the pieces before attaching). 3. Different formulas of PMC can be mixed together in the same piece (PMC standard, PMC+, and PMC3); HOWEVER, make sure you fire the piece so that all formulas of PMC are fully sintered. For example, if I made a piece using PMC Standard and PMC3 I could not torch fire it, I would have to fire it at 1650 for 2 hours. I'm wondering if this might be what's going on with your pieces breaking ... maybe you're using several formulas of PMC in a piece and not allowing all of them to fully sinter. 4. If you really think that your kiln is the problem you can go to a ceramic supply store and get something to test the temperatures that your kiln is reaching (sorry, the name of this product totally escapes me at the moment). 5. Make sure you brass brush & tumble your piece. Use extreme caution when hammering fired PMC (I've broken several pieces this way, its just the nature of PMC). Make sure you don't use a spring-loaded center punch to make the divet for drilling holes in fired PMC because that may also shatter the piece. Good luck! Please let us know what happens! Mollie

Chris - I have no idea. I'm not really familiar with the speedfire cone system, I've only ever kiln fired or torch fired PMC. Hopefully someone else will have an answer for you!

Best wishes,
Mollie