Depression/ cracks in ingot

[purplepepper8]

Hey, Ive been struggling with this for a long time and haven’t found a solution- i keep getting cracks in my ingots while rolling. If you look at the images attached, you’ll see that the ingots always have these kinds of depressions/ crack like things when i pour them. The cracks show up maybe two seconds after pouring once the metal cools a bit. I have no idea why this is the case. Ive also noticed that the ingot ’sweats’ some liquid when annealing after the first few passes through the mill (I’m guessing the depression/ cracks result in water getting trapped inside, which then releases upon heating).

My process is as follows- I heat up the ingot mould till the oil on it starts to smoke, and then move the flame over to the metal. I heat that till it is liquid, throw in some boric acid powder and swirl it around, and then pour into the mould. I then quench it in water, pass it through the rolling mill, and anneal (by heating till red, letting it become black before quenching in water again) and then continue rolling until i reach the desired thickness.

Now most of the time (Id say 4/5 times) cracks form on the sheet. for whatever reason the 1/5 times it rolls out ok (even though the depressions you see in the image are always present). Ive tried different ingot moulds (iron mould, graphite mould), pouring slowly/ quickly, heating for a long/ short time after the metal has become liquid, using a hissing blue flame/softer flame, I’ve tried different alloys and karats. Ive also tried hammering the ingot a bit before annealing and then passing through the mill. The one thing ive found that usually reduces the size of the depression is if i hold the flame on the ingot for a few seconds after pouring. But even that doesn’t get rid of it completely. Ive tried tens of times and haven’t been able to figure a solution and im hoping someone here can help me out. Thank you

 

[silvermon]

Your ingot mold is too cold. You need to place it on a hotplate at about 350-450 degree F. If you preheat the mold with a torch before melting the ingot, it will cool too much. The mold/ingot size you have to use for the mold to be big enough to reliably retain the preheat is closer to that used for a one pound ingot. About a ten point mold weight. A small ingot like shown in the photo is poured into a mold too light weight to retain the heat long enough. Generally, for an open face pour, steel versus graphite mold isn't a big difference to gold or silver alloys the way it is to glass blowers.

An alternative is to use a larger torch (propane is easier here) that can maintain the preheat on the mold while heating the crucible directly (or lined up at a slight downward angle) over the mold. It works, but the coordination can be difficult.

 

[purplepepper8]

Your ingot mold is too cold. You need to place it on a hotplate at about 350-450 degree F. If you preheat the mold with a torch before melting the ingot, it will cool too much. The mold/ingot size you have to use for the mold to be big enough to reliably retain the preheat is closer to that used for a one pound ingot. About a ten point mold weight. A small ingot like shown in the photo is poured into a mold too light weight to retain the heat long enough. Generally, for an open face pour, steel versus graphite mold isn't a big difference to gold or silver alloys the way it is to glass blowers.

An alternative is to use a larger torch (propane is easier here) that can maintain the preheat on the mold while heating the crucible directly (or lined up at a slight downward angle) over the mold. It works, but the coordination can be difficult.

Hi Silvermon, thanks for your response. Currently I do position the crucible in such a way that the end of the flames from the propane torch hit the mould. Maybe its still not retaining heat, I don't know. Ive seen videos where people do what I'm doing and the mould stays hot enough. They also use small graphite moulds like the one I have. The iron mould is fairly large and heavy (my guess is its about 1.5 kilos, or 3 pounds). Shouldn't that be able to retain the heat? I was also wondering, do you think id have more success with a closed mould? Someone else just told me that these depressions are common in an open mould pour.

 

[silvermon]

Hi Silvermon, thanks for your response. Currently I do position the crucible in such a way that the end of the flames from the propane torch hit the mould. Maybe its still not retaining heat, I don't know. Ive seen videos where people do what I'm doing and the mould stays hot enough. They also use small graphite moulds like the one I have. The iron mould is fairly large and heavy (my guess is its about 1.5 kilos, or 3 pounds). Shouldn't that be able to retain the heat? I was also wondering, do you think id have more success with a closed mould? Someone else just told me that these depressions are common in an open mould pour.

Open molds are fine. The best would be, if you can get a large split mold, it can make a small ingot, but be heavier to retain the heat. When I started silversmithing (45 years) I used the same technique you are using, with the same inconsistencies. The first correction I made was to increase my torch size. That helped. As I got better, I started pouring larger ingots. Mostly gold alloys, but many custom silver alloys too. I found that the larger the pour, the easier it is to control all the variables except the preheat on the mold. Even with a larger mold, I would sometimes have a problem. I was buying a new hot plate to replace an older one that still worked. I thought I would try it to help retain the mold heat after initial heating with a torch. It works great, and is the only technique I use.
I would also advise that you don’t over oil the mold. I don’t know if you are, just seen that as a common mistake. I usually just wipe the split mold with a paper towel dipped in old oil. Olive seems to work fine, as does quenching oil.

 

[Mike Cirelli]

I agree with silvermon. This type of mold will be more efficient for what you are trying to accomplish. You are doing it right oil, heat and pour. Open ingot molds are much harder to control temperatures. You will usually always get that shrinkage in the middle with an open ingot mold.

 

[rweigel]

I cast small amounts of gold for rolling in a piece of wood, where I have cut a depression in before. Due to it‘s high surface tension, gold will form a rounded top and ends anyway. The wooden mold is reusable a couple of times, getting a bit larger each time. No preheating required, free protective gas generated by the wood.

The sunken spot in the middle of your ignots ist the last bit that was liquid. This ist also where holes are most likely to occur. If you use a mold of the style that Mike Cirelli posted, this part will be at the top of the ignot and could be cut off before rolling.

Depending on the precise alloy you use, you have to anneal the material after a certain cross section reduction. Usually, this is somewhere arround 30-50%, higher for fine gold.

Beware of impurities in your gold, the tend to cause cracks as well, whe they seggregate between the grains.

Are you using a torch with oxygen? Too much oxygen in the flame makes gold an silver alloys also brittle, due to copper oxide within the ignot.

Good luck

Ralf

 

[purplepepper8]

Open molds are fine. The best would be, if you can get a large split mold, it can make a small ingot, but be heavier to retain the heat. When I started silversmithing (45 years) I used the same technique you are using, with the same inconsistencies. The first correction I made was to increase my torch size. That helped. As I got better, I started pouring larger ingots. Mostly gold alloys, but many custom silver alloys too. I found that the larger the pour, the easier it is to control all the variables except the preheat on the mold. Even with a larger mold, I would sometimes have a problem. I was buying a new hot plate to replace an older one that still worked. I thought I would try it to help retain the mold heat after initial heating with a torch. It works great, and is the only technique I use.
I would also advise that you don’t over oil the mold. I don’t know if you are, just seen that as a common mistake. I usually just wipe the split mold with a paper towel dipped in old oil. Olive seems to work fine, as does quenching oil.

Thanks silvermon i’ll try these things you've suggested. My torch size is already huge. I also barely use oil if at all because I havent had problems with getting the ingot out of the mould

 

[Sinterklaas]

When you are annealing and it starts to sweat it means you are already starting to melt your metal. It is becoming almost to hot. Try to stay under the sweating temperature.

You dont have to anneal if you have good metal and good technique. Next time before you start make an arrow with a sharpy/marker on your metal. Now ALWAYS keep the arrow side up and the arrow towards the rollingmill. Never turn it upside down or backwards. Same side always goes first through the mill. That way you can roll your piece down from a thick size to thin without annealing.

When you have a big piece to anneal it is much harder to get the metal evenly annealed and quenched. So some parts of your metal will be harder than other parts this can also cause cracks to form. Small cracks on the side is not so bad. You can file or saw them away.

 

[purplepepper8]

I agree with silvermon. This type of mold will be more efficient for what you are trying to accomplish. You are doing it right oil, heat and pour. Open ingot molds are much harder to control temperatures. You will usually always get that shrinkage in the middle with an open ingot mold.

Thanks Mike I went out and bought myself one of these yesterday so will try it out.

 

[purplepepper8]

I cast small amounts of gold for rolling in a piece of wood, where I have cut a depression in before. Due to it‘s high surface tension, gold will form a rounded top and ends anyway. The wooden mold is reusable a couple of times, getting a bit larger each time. No preheating required, free protective gas generated by the wood.

The sunken spot in the middle of your ignots ist the last bit that was liquid. This ist also where holes are most likely to occur. If you use a mold of the style that Mike Cirelli posted, this part will be at the top of the ignot and could be cut off before rolling.

Depending on the precise alloy you use, you have to anneal the material after a certain cross section reduction. Usually, this is somewhere arround 30-50%, higher for fine gold.

Beware of impurities in your gold, the tend to cause cracks as well, whe they seggregate between the grains.

Are you using a torch with oxygen? Too much oxygen in the flame makes gold an silver alloys also brittle, due to copper oxide within the ignot.

Good luck

Ralf

Thanks for your response Ralf. I got myself a closed mold like Mike suggested so will try with that.

I don't think it has impurities since I used only fresh 999 gold.

I think one of my issues might be too much oxygen since the flame was quite hissy

 

[purplepepper8]

When you are annealing and it starts to sweat it means you are already starting to melt your metal. It is becoming almost to hot. Try to stay under the sweating temperature.

You dont have to anneal if you have good metal and good technique. Next time before you start make an arrow with a sharpy/marker on your metal. Now ALWAYS keep the arrow side up and the arrow towards the rollingmill. Never turn it upside down or backwards. Same side always goes first through the mill. That way you can roll your piece down from a thick size to thin without annealing.

When you have a big piece to anneal it is much harder to get the metal evenly annealed and quenched. So some parts of your metal will be harder than other parts this can also cause cracks to form. Small cracks on the side is not so bad. You can file or saw them away.

Thanks for your response Sinterklaas. Its not sweating in the sense you're thinking- its some kind of moisture coming out of the metal. I think there may be some micro cracks/ spaces in the metal when pouring which fill up with water/ alcohol when quenched. I think as Ralf mentioned above, the flame might have too much oxygen.

I make sure to always put it in the mill same side up, unless annealed in between.

I’ll try focusing getting an even anneal, even though I think I am achieving that since it all goes to the same shade of red when I do