What causes porosity in PFM ceramics?

[nzbroadbean]

Most of the ceramics I have built before have been on zirconia or lithium-disilicate cores.
Today I had a bench test for new job. In the glaze firing of a PFM I was asked to build a 2mm bubble came up in the ceramic popping off the opaque and dentine.
In the past I would have said the problem is something in the metal that is coming out because it was not degassed properly.
The people at the lab suggested that it was probably because of some crap in the ceramic. They also suggested that it could be that the ceramic furnaces need purging.
I have never seen any porosity of this type in full ceramic crowns before.

What causes porosity in PFM ceramics?

I am using Ducera Kiss for the first time what is the best way to fix this without over rounding my anatomy? (opaque then correction with one firing?)

Your kind help is appreciated.

 

[Gita]

might be very small hole at the metal coping. this might not visible, try using sort of microscope.... or in some area the coping is too thin (nearly zero)... ???
impurity of the metal (if u don't use completely new metal)??
opaque layer too thick??? problem in the opaque???

i've been facing this problem often and so far problem is only one of the above....
i never use ducera kiss...

 

[paulg100]

"In the past I would have said the problem is something in the metal that is coming out because it was not degassed properly."

If the bubble if starting from the frame up then its either contamination in the metal or surface contamination on the metal before your opaque layer.

maybe didnt de-gas at high enough temp (at LEAST 50c above highest ceramic temp)

or didnt clean frame properly after de-gassing.

Or very heavy oxide layer didnt sand-blast off.

just some ideas.

 

[NicelyMKV]

Could be the opaque as well if
your using paste. Paste can be troublesome if you don't fire it exactly as it needs to be.

 

[Tom Moore]

This can be caused by unburned carbon based fuel being forced into the alloy from a torch not properly set. Usually very close examination can narrow down the possible causes. A bubble cause by a stray contaminate in the ceramic can usually be opened up and fixed but one from a contaminated alloy will just get worse.

A bubble that is still attached our in close proximity to a spot on the opaque is usually a start over.

 

[nzbroadbean]

Thanks for the help.

Second day of bench test today. The opaque was done for me so I guess I need to be a little diplomatic. I have enough time to strip it and re-layer it so I guess that will be the go. Otherwise I will spend another two firings sending me ceramic all glassy and round and have a crap discoloured patch.
Work off the metal surface with AlO2 stones degas properly, lightly removed oxide layer (it will grow further in subsequent firings) mix the opaque to a good consistency and prey to Michael Buble, god of bubbles.

 

[charles007]

Forget about patching a bubble..bubble there to stay, bubble in metal likes porcelain, and keeps coming back for you to see bigger bubble.
Strip porcelain off, refinish metal and sandblast frame to opaque..

 

[rshark]

In my experience and training (17 years as Heraeus fixed pros trainer) almost all bubbles come from metal contamination, and incorrect treatment. There are a host of questions I always ask before giving any diagnosis, but the most common causes are contamination at the trimming phase and also the sandblasting phase. I don't recommend using alox stone stones, since they are prone to contamination especially since it is very easy to burnish the alloy into microscopic folds, trapping contamination underneath. Nor do I recommend diamond since small pieces can come off the burr and and be trapped in the metal. Best is a good sharp tungsten carbide. There are many different cutting designs for these burs these days tuned to trim hard and soft alloys. Another regular contaminate is poor quality alox sanblasting media. NEVER use recycled alox, and be careful not to use one that is too fine. I recommend always using 125 micron for all alloys and vary the pressure depending on which alloy. 2 bar for soft and 4 bar for hard. Also check your furnace vacuum pump is up to scratch. An inefficient pump can over oxidise the alloy on your degass fire.

hope this helps

 

[disturbed]

research heat zone, add cooling vents. Porosity will form during the cooling phase after casting. It is MOST important that the coping cool first allowing the porosity to move to the button away from the resto. I have no experience with vacuum casting but when using centrifugal it is very important to have a reservoir with more mass then the resto to prevent porosity.

If you pop the bubble and see bare metal you can pretty much guarantee that it is not the fault of the ceramist and that the metal has issues. Recommend degassing several times but after stacked you have little choice but to strip and re-build. On the plus side you have probably sucked out all the problems with your firings.

An old trick I was taught when potential porosity presented itself during metal finishing was to put a wash of opaque on the metal to draw out the porosity, degas, sandblast and start again. Works well but it's better to make sure the resto's are out of the heat zone and add cooling vents to prevent porosity.

Another quick fix that has sometimes worked for me is to open the whole and very carefully, with a sharp brush, re-opaque the area, dry it in the muffle or fire it at a low temp (100 degrees less than) and re stack the hole. Risky and bad form but it works and if it is done carefully (under scope) is undistinguishable. Make sure the opaque does not wash up the sides and that it is thick enough to prevent having a grey dot. Also helps if you choose not to fire (recommend firing) the opaque to use a dryer mix of porcelain or it will mix with the opaque and look like Sh1t.

Bad form... strip, degas and re stack..

Nah, fix it, save time..

do it right....

do it now, send it out.......

I see a white padded room in my future...

 

[rkm rdt]

What gas are you trying to get rid of disturbed?

 

[Alistar]

research heat zone, add cooling vents. Porosity will form during the cooling phase after casting. It is MOST important that the coping cool first allowing the porosity to move to the button away from the resto. I have no experience with vacuum casting but when using centrifugal it is very important to have a reservoir with more mass then the resto to prevent porosity.

If you pop the bubble and see bare metal you can pretty much guarantee that it is not the fault of the ceramist and that the metal has issues. Recommend degassing several times but after stacked you have little choice but to strip and re-build. On the plus side you have probably sucked out all the problems with your firings.

An old trick I was taught when potential porosity presented itself during metal finishing was to put a wash of opaque on the metal to draw out the porosity, degas, sandblast and start again. Works well but it's better to make sure the resto's are out of the heat zone and add cooling vents to prevent porosity.

Another quick fix that has sometimes worked for me is to open the whole and very carefully, with a sharp brush, re-opaque the area, dry it in the muffle or fire it at a low temp (100 degrees less than) and re stack the hole. Risky and bad form but it works and if it is done carefully (under scope) is undistinguishable. Make sure the opaque does not wash up the sides and that it is thick enough to prevent having a grey dot. Also helps if you choose not to fire (recommend firing) the opaque to use a dryer mix of porcelain or it will mix with the opaque and look like Sh1t.

Bad form... strip, degas and re stack..

Nah, fix it, save time..

do it right....

do it now, send it out.......

I see a white padded room in my future...

I've been told to this. It works(sometimes). We actually have a low-fuzing porc kit laying around and it gets used about once a month(sometimes more) to do this.

I hate it and give a lot of grief to the powers that be for doing it this way, all because the metal finisher got lazy, mixed up his abrasives, forgot to do this, forgot to that, ect....

Then I get to spend an extra hour dinging around and patching crap.

See ya in the padded room....

 

[rkm rdt]

I'd sure like to know what this mystery gas is.Why doesn't it escape during torch casting.Maybe the torch casting is where it "goes into " the metal.
Does that mean you don't have to degass if you cast with induction or milled?
I'd sure like to know what this gas is?

 

[rkm rdt]

I hope no one degasses in that padded room.

 

[TheLabGuy]

I hope no one degasses in that padded room.

It was full the last time I checked. popcorn

 

[Badgerdental]

I heard that you should degas 14 times a day in order to maintain a healthy lifestyle

 

[Tautvis]

Impurities and contamination of a metal, porosity in a metal, grease on a metal surface, maybe layering and porcelain being to dry. Another thing to watch is the correct build up liquid, sometimes the organic particles doesn't burn out properly, need a longer drying time, slower rate.

 

[user name]

Degassing/oxidizing. If you use a torch with a tip like a Harris, take it off and clean the inside. Bang it on the bench and a gentle wipe with a Q-tip, and tap it hard again. They collect carbon on the inside and can shoot it right into your molten alloy.