A998 Color Theory
The color of a ceramic is a science in itself, and unless you are you are in the ceramics business you probably wouldn’t find this topic interesting, but this blog is intended to for both ceramic producers and consumers. We don’t want to get too technical with the details and hopefully you will find this interesting. First, some background info:
High purity aluminum oxide (Al2O3) in full density, sometimes called “high alumina”, ranges from about 94% to around 99% in the technical ceramics industry. It is common to see a 96% and a 99.8% (like Ceramco’s A96 and A998 materials) where the trouble-maker as you might guess is the 99.8% grade. Color varies with the purity level, as shown below. By the way, it is very difficult to get good photos of ceramic materials!
From left to right: 96%, 99.7%, and 99.8% alumina samples
The hallmark color of 99.8% purity aluminum oxide is ivory. No other high purity alumina has this same color, it is unique such that if you go up or down in purity, the color changes. Color therefore must be a function of purity level? Ceramic manufacturers struggle with this color pandemic and I’ve heard a lot of theories on what causes the color variation:
- Furnace plates/setters
- Furnace elements
- Contamination of raw materials
- Dispersion/mixing issues
- Binder issues
- Density issues
- Particle size issues
- Astrological configuration when the ceramic was made
- Process moods
Questions remain. Is the problem specific to a process, or should any process be able to make ivory A998 without issue?
The three plates on the left have some white and ivory (mixed shades). The plate on the right is a “control” with uniform ivory color.
Color as a Function of Purity, A998 Still
We may have an explanation for why our A998 is coming out ivory currently. A customer told us the material was tested and had purity of 99.6% which means we are in the 99.7% neighborhood. 99.7% alumina was made a few decades ago, sort of a gimmick in my opinion, as a high wear type of material – and it’s color was peach. The test on our material may not be very accurate, however, since our material is not peach at all but it is showing a nice deep ivory. I predict that we are in the 99.7XXX% purity range still (this can vary with raw material lots) and the test is probably not accurate enough. We don’t have the data.
Ceramco has been in business for 30 years now and a year or so (like when this blog was started) isn’t a very long time, at least for a ceramic company, and the issues we’ve had with our A998 material have been numerous but perhaps not severe enough to draw much attention from outsiders. Except customers, of course. In most cases, white A998 has been the red headed step child and even caused companies to seek alternate materials to have parts made in, but there is the occasional customer that actually wants the white A998 instead of the ivory… accusing the ivory of being “under fired” and the white being the ideal material. Ignorance is bliss I guess!
It was intended to blog about every theory of why white A998 material was showing up uninvited and the corresponding inconclusive tests that were run but it’s hard to go back over now that we think we’ve cracked the code. However a quick review of each couldn’t hurt, right?
The pedigree of your starting raw materials to make A998 is crucial. Manufacturers aren’t going to lift their skirt on this topic but most companies do their own powder processing anyway to get the results they need, however the alumina has to come from somewhere. The less powder processing you do on your own, the more you have to depend on the supplier, and it goes without saying that the quality of your raw material will influence the quality of your end product. Powder processing includes but isn’t limited to milling (grinding powder), sifting, spray drying, and additives. At Ceramco we depend a lot on the raw material supplier and have to carefully characterize powders and select the best ones available because we don’t have much powder processing capability. A theory I’ve heard about alumina with regards to the rogue color variation in it has to do with trace elements in the material, like ceria (cerium oxide), which is a naturally occurring element and well – depending on where the starting material (like bauxite) was mined from could vary just like anything mined from the earth. Kind of like the character of a beer comes from the water used, alumina may have character depending on the starting material.
Talking about trace elements is almost pointless since can’t really do anything about it and production facilities won’t have the time or energy to nitpick. We’ve tried using aluminas from many different vendors and the quality and service varies, especially if you are a small player in the game. Don’t get me started on that, let’s look at the processing:
After we’ve done our batching/raw material prep work specific to our process the A998 is coming out white, we’ve looked atour alumina and can’t find anything wrong, so maybe it’s something we are doing to it (or not doing to it)? We had a theory about the age/shelf life of our material. Maybe a batch was sitting in the machine too long and some separating of the mix was happening? Yupp, we could observe separation happening but even the non-separated mix exhibited the same white end product, so back to the drawing board. We ended up trying half a dozen things involving the shaping method we use and basically arrived with inconclusive results and more questions.
Moisture, or humidity rather, is an interesting topic. Anyone lucky enough to have a controlled environment manufacturing space has no idea what a seasonal shift means but for those of us that see cold dry winter weather along with hot muggy summer weather know what I’m talking about. In an effort to keep humidity from hitting our parts we installed an air dryer… let’s just say it was worth a shot and the jury is still out as to whether it defeated the seasonal affect.
Firing in the Furnace:
We don’t call them kilns. They are furnaces. Kilns are in your pottery hobby shop. Furnaces are the technical ceramic work horses. Just thought I’d make that clear.
There’s a lot going on in a furnace at high temperature to say the least. We use electric furnaces exclusively but you can get gas fired ones and there’s a lot to be said about the difference between each. Let’s just cut to the chase and discuss density with the simplistic view that the function of the furnace is to fully fire (sinter) ceramics. You have to go to high enough temperature to sinter A998 properly. We’ve looked at furnace elements, furnace plates, furnace furniture (isn’t it weird how words are so close? Seems like the word furniture should be exclusively used in furnaces, doesn’t it?), covering the parts, firing pads, “seasoning” the furnace, part location in the furnace, and firing schedules. Wow have we been busy now that I think back on it! Every time we opened the furnace door, it was like Christmas morning looking under the tree to see what Santa brought (and then the following curse words when you find white parts, but you asked for ivory parts).
The ivory color comes from the color centers of the crystal structure of the alumina. Thus we have to make sure the alumina is fired to the proper density. There’s a trick to doing this, and it’s pretty well known but perhaps not appreciated enough because we knew it all along. Indeed there’s a very technical explanation and I promised not to go there so I guess we stop here and move on to the many other exciting projects we are working on.
Update – Gas vs. Electric
Thanks to some advice from several experienced ceramic persons we have more insight into the control of color in our A998 material. It may have been mentioned before that using gas fired kilns versus electric can influence whether ivory color graces us with its presence or not. What wasn’t understood before is why. Well, it comes down to oxygen content, actually. When you run a gas fired furnace, the combustion of the gas fuel produces airflow and reduced oxygen inside the furnace box. Electrically fired heating elements however are not going to do either of those things (make airflow or reduced oxygen). It’s apparently common in the industry to use gas fired furnaces to get the right color, i.e. those who have gas firing capability have not found the color of their high purity alumina to be an issue, or put even another way: if you are firing your 99.8% alumina with electricity and having color problems, try gas.
Well, we don’t have any gas firing capacity in-house. So we fooled around with the air purge function of our electric kilns to try and mock airflow and oxygen content of a gas fired box. The furnaces have a fan that is already installed which takes air from the outside and purges it through the box and exhausts out the top/chimney that has a mechanical hatch. Turn the fan on, the hatch opens up, the air goes through the firing area and out the top. We can control the speed of the fan with a potentiometer type dial on the furnace control panel.
Experiments were run, results are in, and color is good! While there is room for improvement and better understanding, it would be done best by using an oxygen content monitoring device and we don’t have one, so theory will have to be good enough for now.