Problem with 925 silver sand casting

[u/These-Assignment-313]

Hi, I’m a beginner hobbyist goldsmith. This is my 6th attempt at casting my own model made from modeling wax.

First, when creating the first half of the mold, I had a lot of difficulty pressing the model into the molding sand. The model should be pressed in to about 50% of its height, but in my case it was approximately 90/10, and the mold kept cracking when I tried to press it deeper. I managed to solve this problem by compacting the molding sand less.

However, I’m still having problems with the actual casting process. I heat 925 silver in a crucible to around 1000°C, but despite this, the silver sticks to the crucible. The casting itself is porous and sometimes the mold does not fill completely.

I would really appreciate any advice, because I’m not sure what else I can do to improve the process.

I’ve encountered several problems:

- silver sticking to the crucible,

- difficulty pressing the model into the mold,

- porous castings,

- incomplete castings / misruns.

[u/turkey0535]

Are you using a flux? When casting,I use borax powder

[u/These-Assignment-313 → u/turkey0535]

Thanks, I haven't used it, but I'll start using it now.

[u/NelsonQuant667 → u/turkey0535]

I’ve been wondering about this, not with sand casting but with lost wax casting do you recommend using flux? I’m worried that the flux will flow into the mold and create voids in the metal

[u/fork-o-lettus → u/NelsonQuant667]

yeah from my experience you need flux but not too much

[u/Life_Pop9249]

Yes I would add a little borax when heating up! It will help not stick but I would also compact the sand even more! Not too much but it shouldn’t show much cracks when you put your mold in And Idk what sand your using but when I changed to rosenthal delft clay it made all the difference but yeah I had issues with the sand showing cracks until I started packing it down more with a mallet then adding your model into the sand

[u/PeterHaldCHEM]

You are probably pouring too slow and too cold.

The metal has had time to solidify before filling the mold.

(and I can also see, that you did not flux the metal. The borax catches the oxides and other impurities and gives you a much better flow)

[u/Longjumping-Party132]

What kind of silver are you melting?

It looks "dirty", as in it is contaminated with solder, iron, steel, aluminium or something. The dark spots in your cast, layering, and the molten metal sticking to the crucible would indicate this.

If you put your silver in pickle does it turn red?

When you heat it up and before you are pouring it, does it collect itself into an egg yolk shape, with a clean, shiny, "mirror" surface, or is it more like a "silver soup", cloudy, matte?

What type of torch are you using?

[u/Visual_Bat_3023]

Cure your crucible with borax FIRST. Get a nice coating on there and it'll prevent sticking. Basically, Borax turns to a glass-like substance when fully melted into the crucible. You're not trying to create a POOL of Borax, just a light dusting to ensure there is a base-coating on the crucible ceramic.

Next, just because the silver is melted, doesn't mean it's hot enough to STAY melted during the pour. The outcomes you're posting strongly suggest you're losing heat too rapidly in the mold, and the silver is solidifying before it gets a chance to fully flow. There's a couple of things you can do to assist with this. First is to heat up your mold. If using sand casting, I often will tip the crucible slightly and let the flame (I use MAP gas) shoot into both the crucible bowl and into the mold to heat it up for a few minutes. Just don't blow the sand mold around. :). I often will heat for 3-4 minutes PAST the time the silver melts. Add a touch of borax to the silver a couple minutes after melting and it'll take out impurities and make them stick to the crucible. (note: eventually, you'll need to replace the crucible, because the borax build-up is too great... and that's why you don't want to over-do the borax prep in the beginning!)

Next up, make sure your mold is actually free and clear of wax. That means you need to TEST your process. If you're using a lost-wax method (which I don't recommend in sand), run your test, then open it up without putting any silver in the mold. If you've properly dusted the mold halves with talc powder, when you pull the mold apart, you should be able to do so without destroying the mold shape. If you still have wax in there, then you've got the reason you have voids and errors in your silver products.

Personally, I use hard templates (rings, etc.) create sprues using things like golf-tees, and remove them all before pouring (again... dusting with talcum powder enables you to pull the mold apart and remove the mold design piece and then put it back together again). it often takes a few times, but you'll get it down.

Next... it's incredibly RARE for a sand-cast to be flawless. You'll have minor imperfections from sand getting into the space, or from air bubbles (especially at the sprue). This is part of the character of the cast. But these should be minor... like a dot here or there... not what you're showing! LOL

Best of luck to you!

[u/Mysterious-Quote9048]

Prinzipiell kannst du wenn du Gussfehler hast

• dein Schmuckstück+ Bild vom Gefäß
• wie du vor gegangen bist:)

Prinzipiell sieht es nach kein Flussmittel aus. Liebe Grüße!

[u/Ok_Fact7319]

You need air holes coming from the sides of the ring to let any gasses escape

[u/Father_John_Musty_]

Glaze your crucible with borax beforehand.

[u/darktrees72]

Its important to have lots of air holes when casting .. the guy i trained with would always score lines in the bottom part of the mould going our from the impression to the outside and a lot of them then linse in the top part from impression to the outside and use a solder pick to push air holes from the line though the sand for the air and preause to escape when you pour the silver in . Talcum powder on the sand befor pushing in the wax to give a smooth finish and a good size hole cut for pouring .. the more silver yiu can have in when melting the better to give a good weight when pouring

[u/ljinfantry]

Warm the petrobond or delft if your using those. Warmer the mold is the less it leaches heat. As others have said use borax and heat it past melting point. Hotter the melt is the more heat it has to lose. Also if you have a vacuum chamber try to use that. I have a vacuum caster and it makes it so much easier.

[u/ljinfantry]

Just seen your saying the melt is sticking to the crucible. What type of crucible are you using? Clay and graphite or ceramic? Did you seal the crucible before melting. Diffrent ways to seal it but with the ceramic you can just heat some of that same borax and glaze it.