Full Cast Gold Investment Question

[Travis]

I have been having a hard time dealing with porosity lately. I believe it might be my investment. I am currently using Microstar HS for all my alloys. I know all the rules on the spues, sprue gauges, location in the ring etc. I think maybe my ring is too hot when I cast, 1400 degrees F. I have been getting a lot of FCG lately and would like to fix this. The gold content is 55% and 77%

I would like to stick to ring less if at all possible.

Any help is appreciated.

[Brian8]

Travis,

1400 is WAY too hot for FCG!! Most manufactures suggest the burnout temp to be around 850 F. But we've found that to usually be just a bit too low. So we typically set our oven at 950. But if you're using a 77% AU then I'd even consider setting it at 900 F.

The problem with casting gold into too hot of a ring is that due to the golds low melting temperature, if it's thrown into a HOT ring, the gold stays in a molten state for too long of a period, and the heat of the metal, itself, can cause an internal boiling effect. If the porosity you're experiencing runs deep and at different levels, i.e. you discover more and more as you polish back, then this is most likely your culprit. Try setting the burnout temp lower and let us know your results.

Brian

[Travis]

Microstars instructions say to take the ring to 1600 f for 35 minutes. When I cast at 1400 the ring looks charcoal black, looks like its not ready. If I take it to 1600 for 35 minutes then back it off to 950 the program would take 5 hours. I am not sure about the time but I will run the program tonight to see. I thought there would be a better investment just for fgc.

[Brian8]

Travis,

When we cast our FGC the ring is black we only take it to the 950f.

The past experience showed us then when we raised,then lowered the temperature the ring would expand changing the dimensions of the crown. So we tried to just leave it at 950 (it remained black) and everything worked out... and we have been doing it ever since.

I would not follow the investment suggestions but the metal company's suggestion because they are ultimately your metal expert.


Brian

[Travis]

Thanks so much, this will save me headaches tomorrow when I go to finish them.

[Brian8]

Travis,

No problem I hope that this fixes the problem... if not let me know.

Brian

[JEmirzian]

I cast PenCast 63 by Pentron and we burn out at about 1250 F but let the ring-less investment come back down to 1000 F before casting. Been using Whip mix Formula One. I also sprinkle a bit of borax based flux on the metal just as it starts to melt and have had GREAT results.

How far is your reservoir away from the wax pattern when you screw...I mean sprue? And what guage sprue are you using?

[Travis]

I finished the two crowns today with great success. The ring was black and still smoking when I took it out of the oven, no guts no glory, it was a stressful moment but it turned out nice. I hate re waxing, usually because perosity would be too bad. I am getting my copings at .4mm and now have my fcg problem resolved. Thanks guys!

I use zahns direct or indirect sprues, I don't know which it is, don't have it in front of me. It is 8ga with the reservoir in the middle. So putting it on the crown would always be the same distance.

[Al.]

Originally Posted by Brian8

Travis,

1400 is WAY too hot for FCG!! Most manufactures suggest the burnout temp to be around 850 F. But we've found that to usually be just a bit too low. So we typically set our oven at 950. But if you're using a 77% AU then I'd even consider setting it at 900 F.

The problem with casting gold into too hot of a ring is that due to the golds low melting temperature, if it's thrown into a HOT ring, the gold stays in a molten state for too long of a period, and the heat of the metal, itself, can cause an internal boiling effect. If the porosity you're experiencing runs deep and at different levels, i.e. you discover more and more as you polish back, then this is most likely your culprit. Try setting the burnout temp lower and let us know your results.

Brian

I have to disagree.
Most metal manufactors have two burn out temp recomendations. One for Low Heat investments which is between 900 to 1000. The other for High Heat investment which is between 1200 to 1400.
If you are using a High Heat investment but only taking it to 1000 it is black because you are not burning out all the gasses.
Casting temps of most FGC alloys is around 1800, putting that into a 1400 ring is not to hot.
I use CB 30 investment. I burn out at 1600 for 1 hr. Crack the door half way till the temp drops to 1450. Close the door and give it 10 minutes then sling the metal. It adds about 15 minutes to the burnout process.
Ive been doing this for about 15 yrs. I get consistant fits and no porosity.
Assuming that you are not overheating your metal your problem is more then likely an undersized reservoir on your sprues.
I found that the preformed sprues do not have a large enough reservoir for many of the crns I do weekly, and I ALWAYS get porosity on these crns where the sprue is connected, and if they are really large it is over the entire crown.

What I do for about half my FGCs is make my own. I sprue it with a short 10 ga sprue, then I take rope wax and roll a ball/reservoir about the same size of the crn than attach that, then add a short 8 ga sprue on the reservoir to attach to the sprue former.
It takes me about 30 seconds to sprue this way, but its alot quicker than remaking the crn because of porosity.
It also makes a large button.
Ill bet a steak dinner that Im right.
Al