How to get a nice round top on silver?

I just started hand pouring silver. I use an electric furnace and a graphite mold. I heat the graphite mold pretty hot. It glows slightly orange and then I remove the MAPP torch and then pour my silver.
Randomly, I get lips on the top of the bar where the silver has sort of climbed up the mold and hardened that way. It isn't consistent with the direction of the pour, I don't think. I am not sure how to consistently get a nice rounded top to my loafs. Any help would be appreciated.

melt the silver in the mold under CO2 atmosphere.

I was under the impression that would change oxidation and discoloration of the surface. I just wanted smooth edges. It is as if something overcomes the surface tension of the silver in areas (inconsistently) and the silver being poured doesn't rebound and some of it "sticks". I had the impression that maybe it was a temperature thing or something but I felt like I was getting everything hot enough.

The CO2 would help to keep the silver from picking up oxygen during the melt and spitting it out during the cooling process as well as oxidation and discoloration of the surface.

Oh are you talking about the "warting" I have seen on some pours?

I was talking about the edges rising up to the surface or sticking to the surface of the graphite mold as shown in the red circles in the attached picture.

Getting into conjecturing at this point.

CO2 or a reducing environment would also help oxidation which at the surface or edges would give rise to the cooling scale or oxides and impurities.

In my mind, I imagine the cooling of metal in a melt can be similar to the forming of crystals from a saturated metal salt solution, the crystals can push out impurities towards the surface, or at least out of part of the crystal structures which form as salts from a solution or an alloy of metal, many factors can come into play composition of the solution or molten metals and gases, temperatures, cooling rate, and environment, which can greatly affect the crystal structure of the alloy as well as the surface and contaminants...

Silver loves to absorb oxygen in a melt, this oxygen and the base metals involved in the melt react to form oxides which on cooling the silver alloy will push out as the crystals cool.
These salts, impurities or metal oxides will also have different melting points than the alloy itself...

Impurities can act like seeds for crystals to begin to grow or places for oxides to form groups or larger areas of scale, Oxidation of base metals such as copper or iron in the alloy will try to get to the surface, climb the side of the mold edge to get to the surface picking up more oxides...

Not really related, I have very little experience with jewelry but when I have made rings with silver that Had PGM metals I noticed hard spots at the surface of casting (like a small impure lump) on the surface of the casting much harder than the surrounding alloy composition.

While just rambling I notice the red circles are on one end, would this be the cooler or more of the oxidizing range of your torch during the melt???

Ah, now I get what you were trying to say. It makes sense to me now.

Thank you for that insight.

I am not sure how I could create a reducing environment with a simple graphite mold in an open to the air setting. But now I have something to read up on.

I don't think the raised edges would be towards a cooler spot on the mold. I will try and pay more attention next time. I think they are mainly on the other side of my pour. I have poured the silver long ways and short ways in a rectangular mold and it seems like it is the opposing direction from my crucible is where the protrusions form. Maybe the extreme heat of the crucible does keep one side warmer during the pour.

Covering the melt in charcoal (or as an addition to flux) along with using a reducing flame of the torch or furnace burner(fuel-rich/oxygen-poor, yellow flame) will help to absorb oxygen, giving a CO2 reducing atmosphere...

I'd go for hotter. That edging tends to come with my pours when it isn't hot enough.

Edit- a picture tells a thousand words as they say. These are straight poured rough silver from cementation before they go into a cell. This is the result I get from heating with Oxy Acetylene and making it very hot before pouring- no "edges."

Hope it helps in some way.

Jon

+1 on the metal not getting hot enough before pouring. The good news is you can put that ingot in the melting dish and heat it to just when it starts to go molten and it will smooth the corners. Hit the ingot with the flame from the top.

Thanks for all the info. You are all very helpful

I will post back after the next set of pours to see if I made improvements.

Well, I am getting better. I am using a crucible right now to try and eliminate carbon contamination.

I am getting smooth sides and bottoms with a nice rounded top. I think the key was to get to mold to ~450C. I also am making sure to keep the silver really molten and rolling.

The last obstacle I have now to getting a pour worthy of keeping, is the crap on top. I don't know why I am still getting that with a crucible that should only have borax in it. Here is a pic.

Is that brown stuff trapped in borax? The color suggests contamination. The top of the buttons look a little off as well. I could be wrong though. If that's borax just heat the button in diluted sulfuric and it should come right off.

ETA: do you have pictures of your melting dish?

The rough surface looks like spitting. What are you doing to make sure the silver doesn't absorb oxygen while you're melting it?

Dave