Porosity in Silver Casting

Since porosity can be generalize term for any voids in a casting some pictures would help.

Are these defects in the metal at the top as the cast piece sets in the mold? Are they round or irregular in shape?

I will try to post some pictures in a few hours, when I get home.

I think artsmith does this kind of stuff.

"last time I used only new silver (not scrap)" Is this as if to say your second casting is using 'scrap' silver? In which case I'd imagine its to do with getting crap caught up in the melting and casting process

That is why I tried only with new silver. The result was the same. So, I'm doing something else wrong.

Silver bullets for the coming Zombie apocalypse?

Silver is a strange metal, when molten it absorbs oxygen from the atmosphere which dissolved in the metal (not as a compound - true solution). When it cools, the oxygen is excluded creating voids bubbles etc. The solution is to melt it in a vacuum and hold the vacuum until the metal has fully outgassed. Another way is inert gas blanketing and sparging of the melt.

Search here for details on how to deal with the problem

Oxygen solution in silver castings

Here are some pictures.

Some of those look like the metal cooled to rapidly as it was poured. I would increase the temperature of the die a few degrees. That is something you will need to work with making small corrections in temperature. Though you have given the temperature we don't know what it's taken with. How accurate it is. Or the environment it being done in. They all have some effect. It's also hard to tell the orientation of the defects to the top of the casting. They appear to be all over. And irregular in shape is why I say cooling too quickly. If the surface of the casting die is too cool. When the molten metal hits it. It will start to solidify. More like taffy this metal will roll with the metal behind it's force pulling voids into the piece. Also the metal is displacing air which can get trapped as bubbles as the metal poured.

Never cast silver but a lot of lead. Those types of defects would be eliminated by increasing the die temperature. We didn't pour just one at a time as I believe you are. So the first ones went to the side until the die got hot enough to stop the voids. Then worked at a steady rate so that it maintained the heat.

Is the die being heated? If not some preheat to it may be needed. It will help to eliminate both problems. Maybe set the die on a heat plate before pouring.

I use the oven from the picture for dewaxing and the furnace for melting silver. After the dewaxing is finished (the program finishes at 730 Celsius), the die cools down till 470 Celsius. The tempereture is measured with the digital thermometer integrated with the oven. Maybe it is a good idea to take the die at 490 Celsius, make the casting and see what happens.

Those voids look like cooling. You need a couple of seconds between pour and solid and so need a hotter mold. Also, try adding some base and midrange sound around or under the mold when you pour. Really, it helps the bubbles move! Or you can tap the mold downward as it cools which forces the bubbles up and the metal down. This last is why lost wax and a centrifugal feed casting arm are used with gold. A vacuum will help fill the mold, but as others have noted, silver disolves oxygen and that oxygen is comming back out when the silver cools, so the centrifuge or tapping help the bubbles get a direction and move while they can. Keep your mold hotter.

Dentists or Labs for Crowns/partials/inlays use a centrifical pouring process. I do not know if that would help, as they generally pour molten Gold in their lost wax casting technique.

I remember trying to take some old Silver Solder I had acquired to make a 6 inch bell. The bell was awful and I could not exclude major porosity, which I thought was related to sand casting. Bronze worked much better. So did Aluminum.

Hearing now about oxygen attraction in the Molten state makes sense. Just comments, i have no suggestions to help you. Maybe there is a special Flux to reduce oxygen or hydroscopy.

Read the section on Porosity in this article here, which explains the difference between gas porosity in silver and porosity caused by shrinkage.

Your description of voids that appear after polishing sounds like gas porosity, and if that is the case, don't increase the flask temperature, since sulphur dioxide degassing can cause the same problem if the investment is overheated. Instead, try using a reducing flame to cover the melt surface as you pour, as suggested in the Hoover and Strong article above. They give other solutions as well, for protecting the melt surface, but the reducing flame is the best choice - it will also ensure the metal is not cooling too quickly during the pour, and help prevent shrinkage porosity if that is in fact the problem.

Disclaimer: BTW I have never made a casting. I do work with silver, and I've learned most everything I do with it by reading everything I can get my hands on first, then trial and troubleshooting. So my advice is not from experience in this case, unless you count experience reading! _{(insert geeky-looking smiley, mysteriously missing from the menu of CR4)} The experienced contributors are in a better position to judge from the images, what kind of porosity is the problem...

Great answer...yes sir by the pics shown..that is gas porosity..causes?

#1 burnout..was that a carved wax or injection wax?? 2 different burnouts..

remember that carving wax is denser and it will start to melt at about 230/250 f..While injection wax melts at about 160f..all the instructions given with the type/s of investiment unfortunately are based on injection waxes!!:(:(Preheat at 300f...bla...bla...bla...Nonsense

Invest your flask ( Carved wax) ..let set... for small flask 1.5"x2'dia 2hrs larger for 3.5...extra lge ( Personally!!) 6...8 hrs.

DO NOT PREHEAT THE OVEN!!!! Scrape the top of the flask..cold oven..go from 78f to 250f in 1hr...hold for 2.30 hrs..ramp at 180f per hr to 1400f hold for 5hrs...never mind the bs about investiment breakdowns...it is crap!! trust me afterall Kerr investiments were in use before the dental investiments and when you are casting nickle chrome base alloys you shoot the metal in a 1500f flask!! reduce the temperature to the proper casting range depending on the temperature of the casting..Open the oven and check the entrance of the flask(Bottom)Is it chalk white?? If yes you are in business..if you see any traces of greyish brown at the entrance,..sprue chanel?? Please put the flask back into the kiln and raise it back to 1400f or until it is completely white. About my experiennce..Carving waxes suck!! They never burnout clean..God only knows why...

Unless you are casting Platinum like me and many others...different investiment

burnouts at 1800f!!! Believe in me ..nothing survives at 1800f!!!No ash...carbon...

Nothing!!

Torch casting??As per above these are the settings I'd reccomend..

#1 gas/oxy settings..Please keep away from acetylene..Worst gas ever for inexperienced casters!!acety= acetone...GET IT!! Carbon. residue lake crazy!!!

Going back to oxy/ gas..it can be lpg or natural gas...( Natural gas is way cleaner..but no as hot..about 1000f less then lpg)

Regardless both great sources of heat...

Open your gas and set at 10 psi...open your oxy at 20 psi..try to reduce the flame and obtain a flame about 14 '' long and a blue cone of about 1/4...1/2 inch long..please no hissing noise!! .I know it is bloody difficult to explain over a paragraph..but give it a try!!Keep your flame at about 3..4..inches away from the crucible..and of course preheat your crucible to a red hot..this will speed up the melting of your metal..Please never move the flame away and please do not stir the melt too often,,this will introduce gases to the melt!!

Just before all the metal is melted..load the flask ( Keep the flame on the metal..I can see you:-):-)...keep melting ..when into the shiny blob..count 12 seconds and release the centrifuge....White golds? Wait 6 minutes and quench...yellow gold 6 minutes for every 5 grams of melted gold yes sir this is my formula 6 minutes for every 5grams..including the sprues and botton!!Example total 34grams gold..

6x 35=35approx. minutes..quench!!

All red golds should be quenched red hot!!

(and all the fine metals..gold and silver and Platinum..as long as they are fine 9999

They can be quenched immediately!!!)

DO NOT LET THEM AIR COOL! THEY WILL CRACK!! WHY?

Because the molecular structure of gold and copper need to be frozen at the red hot spot otherwise the copper will solidify before the gold ..creating empty spots which can lead to cracks and distortions in the cast

Set your oxy at 20 psi...lpg or natural gas at 10psi..PLEASE NO OTHER GASES !!

Open the gas to a full flame now introduce the oxy..what you want to see is a flame about 12' long with a blue cone about 1/4...1/2 long...the end of the flame should curve upwards!! OK? Heat your metal from approx. 3...4' actually aim the flame on the crucible not directly on the metal and let the heat do the work..You will notice the sl the metal will slowly come into a blob and start to shine, add a small pinch( Like salt) of good brand of boric acid( NOT BORAX!!)I hope all these tips help my friend I have been trying ...to get the best casting for the last 30 years!! Kidding aside..let me know.....best Antonio Loy

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Two things I can think of that not been said but I've not read the links yet.

First is pour speed. A slow pour could let the molten metal cool a little.

Second is the smoothness of the pour. Not splashing it or dripping like a leaking faucet. But a smooth start to finish pour with no stopping/starting.

Just a thought.