YELLOWING?!?!?!

[a3dl]

We have been experiencing a significant amount of yellowing w our porcelain layered zirconium crowns and bridges near the margins...the oven we use for firing and glazing is a metal free oven and only see zirconium...it has ceramic casings for thermocouple... we can not figure out why it happens...the crowns look great but the yellowing at margin area kind of sketches me out when it come to qc... Can anyone help? or has experienced same thing?

 

[TheLabGuy]

I've seen this, but it's because the sintering oven of the zirconia oven needed to be cleaned, had nothing to do with the porcelain oven. The cleaning cycle needs to be run more often to prevent this. Some of the guys on here that sinter all the time can probably be more helpful.

 

[Mark Jackson]

We have been experiencing a significant amount of yellowing w our porcelain layered zirconium crowns and bridges near the margins...the oven we use for firing and glazing is a metal free oven and only see zirconium...it has ceramic casings for thermocouple... we can not figure out why it happens...the crowns look great but the yellowing at margin area kind of sketches me out when it come to qc... Can anyone help? or has experienced same thing?

Call your frame lab, or if you are doing them yourself, your sintering oven is contaminated. There is a decontanimation cycle that needs to be run at least twice a week. Unfortunately, by the time the contamination starts showing up, it's too late to clean it out, but the sooner you can let them know,, the better.

Also, you might want to decontaminate your own ovens too, because whatever is outgassing into your porcelain is outgassing in your oven too.

 

[araucaria]

might be an idea to look at contamination from the immediate environment, what's floating in the air at your place? Aircon, burnouts,smokers,factory next door, etc....

 

[rebel dental lab]

I have had this happen a few times myself. I was using a very used honeycomb tray and pegs and it was yellowing the porcelain at the margin. I sampled some Ceramco PFZ and this happened constantly. You may want to cut the porcelain back to see if goes all the way through to the core. Hope this helps Ferris

 

[DMC]

Alex, walk across the street and get some new heating elements. I have a big stash.

What size/shape heating elemtns do ya need? I fiddle with ovens all the time. You have exposed MOSi2 alloy burning in the oxygen containing atmosphere in your oven. The protective layer of Silica has flaked off and you get flourescent Yellow Zirconia. By baking the oven empty over and over you sometimes get the layer to re-grow. I have a hundred used elements you can come look at tommorow.

Newer style ovens do not have this heating technology. Those go for $25k or so?

Arressa, Lauren, Marc and I talk about heating elements on an hourly basis. After every bake they need a very close inspection. I replace before the zirconia goes Green.

 

[Mountain Goat]

Sounds like bad elements in sintering oven, I agree with ***, one goes bad and you can see it very slightly in the frames/copings, then the next cycle BOOM!!!! YELLOOOOOW! Jeez, if you outsource and are receicving yellowed frames, even just slightly yellow, shame on your milling center for sending that back to you.

 

[DMC]

Nothing is contaminating your oven. It's a reaction between the exposed alloy of the elements and regular air. It makes a nasty gas that stains the Zirconia. Has nothing to do with colored Zirconia, or Lava, or ??

All MOSi2 elements suffer this. The CTE of silica layer is not the best for sticking to Molybdenum. It eventually will flake off. Usually on a Mon morn. after oven is sitting for days, and maybe the building temp drops.

Years ago, this was not understood and people started to blame all sorts of stuff. There is no such thing as a cleaning cycle. It's just a long shot for trying to re-grow the coating on the elements. 50% of the time you may get the element coated again and shiny blue/grey.

 

[Mark Jackson]

Years ago, this was not understood and people started to blame all sorts of stuff. There is no such thing as a cleaning cycle. It's just a long shot for trying to re-grow the coating on the elements. 50% of the time you may get the element coated again and shiny blue/grey.

I disagree. There have been a number of studies about the build up of contaminants on the exposed heating elements, many caused by the outgassing of pigments and binders. Organic binder in isostatically pressed zirconia, oxidize and stick to everything, including heating elements and pourous muffle surfaces. There ARE decontamination cycles, intended to burn off the contminants.

You can't compare your ovens to my ovens, and vice versa.

 

[Mark Jackson]

Another thing that I forgot to mention is the pigmenting process. If you take a zirconia puck that was not designed to be used with a pigment, and soak it, it will react differently than one that WAS designed to be pigmented.

Also, the age of the pigmenting solution, and whether or not it was given a pre-drying cycle is important. Also, microwave sintering will not work well with pigments, and the accelerated 90 minute cycles that some ovens afford will not always work with pigments, unless they are pre-dried.

Surface contamination can also cause problems when inappropriate materials are used for finishing, particularly aluminum oxide wheels and stones.

The bottom line is, if the lab you are using is following the manufacturers instructions TO THE LETTER, including equipment maintence and calibration intervals, and not substituting a single thing, you shouldn't have any problems.

FDA GMP's prevent many technicial problems that can affect color, quality and strength.

 

[harmonylab]

I have the same problem happen now and then, when I forget to run the cleaning cycle.

After a cleaning cycle (or more, depending on how long I neglected),the problem goes away.

 

[a3dl]

its not milling center..it has to be my porcelain...or oven..not sure yet...we are doing some testing b/w different ovens and different porcelain..let yall know how it turns out

 

[jimpa]

Okej, I'm working with Wieland and do the sintering in there owen. I have some times the feeling that the color has been to dark, at least darker than before. There new stuff is fast, 10 sek. The old one, 30 min was better I think, but I am always accurate that it don't stay to long.
Maybe I should run a decontanimation cycle.
Do you guys have the program..? how high is the temp and how long..?

Jimpa

 

[Mark Jackson]

Okej, I'm working with Wieland and do the sintering in there owen. I have some times the feeling that the color has been to dark, at least darker than before. There new stuff is fast, 10 sek. The old one, 30 min was better I think, but I am always accurate that it don't stay to long.
Maybe I should run a decontanimation cycle.
Do you guys have the program..? how high is the temp and how long..?

Jimpa

Every furnace manufacturer should have their own cleaning cycle. I'd call them and ask. Run it two or three times, and vacuum out the oven after each one.

 

[DMC]

Okej, I'm working with Wieland and do the sintering in there owen. I have some times the feeling that the color has been to dark, at least darker than before. There new stuff is fast, 10 sek. The old one, 30 min was better I think, but I am always accurate that it don't stay to long.
Maybe I should run a decontanimation cycle.
Do you guys have the program..? how high is the temp and how long..?

Jimpa

If you are using liquid stain, then it may be dehydrated in the winter, or you left the top off too long. Throw away stains and get fresh liquid. Maybe this helps??

 

[jimpa]

Thank You MARK, The manufacture said run the regular program twice, but didn't say how often...

 

[Mark Jackson]

Thank You MARK, The manufacture said run the regular program twice, but didn't say how often...

We run it every third day.