Non-Chemical Blanking and whatnot

[jazzjunkie]

Hi there,

First post warning! Ive been collecting silver for a while and am planning how to have a shot at refining some scrap into fine silver.

Im not sure if my souce is good or whether or not ive made a mistake, but it seems that not much force is required to press silver (im thinking about pouring a rough ingot in a sand mould, making it the correct weight, then pressing it in a die to make a formed bar of the correct weight with or without embossing)

Going by a yield strength of 1000psi for silver (found it off google)
a bar of 50mm x 25mm would only require 8619N to enter the plastic deformation region for silver... Thats like less than a tonne and ive got a little 2-tonne hydraulic jack.

This seems way too easy! Is the yield strength correct?

 

[element47]

Im not sure if my souce is good or whether or not ive made a mistake, but it seems that not much force is required to press silver (im thinking about pouring a rough ingot in a sand mould, making it the correct weight,

Without really super high quality refinery equipment, I strongly doubt an amateur ingot pourer will have adequate precision control over the amount of molten metal you'll be able to deliver into a mold. If you go look at pictures of older, hand-poured "loaf of bread" ingots, almost all that I have seen that are not Englehard or Johnson Matthey have the actual final weight hand-stamped into them. Even today, COMEX "good delivery" bars of 1000 oz are never exactly 1000 oz. They are weighed and hand-stamped with their final weights. This is not how coins, for example, are made. There are a bunch of videos on youtube which show coining operations. Silver or the metal for the blanks is rolled to a very tightly controlled thickness in long strips and then the planchets (disks) are cut out on a die-press.

...then pressing it in a die to make a formed bar of the correct weight with or without embossing) Going by a yield strength of 1000psi for silver (found it off google)
a bar of 50mm x 25mm would only require 8619N to enter the plastic deformation region for silver... Thats like less than a tonne and ive got a little 2-tonne hydraulic jack.

I think you are off by a factor of at least 50 in terms of being to die-press a chunk of silver. Plus your method will have approximately zero impulse or velocity behind it. You will not be able to get "flow". You may be able to bend of flatten a chunk of silver, but I think you'll just end up with a flattened or bent piece of silver.

I have never, by the way, seen or heard of silver being cast in a sand mold. Perhaps that is a shortcoming on my part, but I've only seen graphite or iron molds being used. I would be very hesitant to use sand, because if there is any flux in your molten silver, it would absorb into the sand, at least in my estimation.

 

[rusty]

Have you considered using a plaster mold.

 

[element47]

"Have you considered using a plaster mold."

Do you know if that is a real-world feasibility? Plaster is hydrated calcium sulfate. Like concrete, I am not sure that plaster *ever* loses all of that hydration; meaning, some some degree of that hydration remains in the apparently dry solidified material. You pour molten metal into such a thing and you could get violent spalling of the mold walls. I don't know. I know if you concentrate a torch on concrete it can spall (split off) with violence, almost explosively. Folks have been working with molten metals for hundreds of years and without trying to sound like a religious zealot, there are reasons why certain things are done in certain ways.

 

[qst42know]

In the jewelery and dental trade the plaster they use is referred to as "investment plaster". Gold, silver, platinum, and non precious alloys are cast in it every day. And anything looses it's moisture at 1500F.

Sand inclusions will destroy your die I wouldn't recommend it.

Apparently there are ways you aren't aware of.

http://en.wikipedia.org/wiki/Investment_casting

 

[jazzjunkie]

I guess correct weight is not such an issue as you can always go for an over-pour and just ensure that there is at least x grams in the bar. My thinking was that its tricky to pour a decent amount of silver cleanly so why not rough-pour a chunk of silver that is approximately the size of the desired ingot then use pressure to form it into a uniform bar?

From my sketcy recollection of materials mechanics the yield strength is the resistance that a solid puts up to permanant deformation and when i did the calcs it came out to be quite a low amount of force required to stress the bar beyond the elastic point... under a tonne, but posts i have read are talking about 26 tonne presses for gold... i just dont understand why so much force is required in practice!

The sand thinking came from the traditional form of casting which seems good to me... http://www.youtube.com/watch?v=K0sCDTRwMwg these rings are clean and excellent pours with precise volumes but i assume freezing is a problem with larger casts... which is kind of what lead me to the idea of cold forming rough ingot shaped casts.

 

[qst42know]

The delft clay video is nice, but anything that freezes so quickly can trap bits of mold material or air. Even hot investment, force filled centrifugally or under vacuum can do the same.

Poured in a durable ingot mold stray mater is avoided and leaves a tamper evident surface. No amount of polishing can reproduce the "as cast" surface.

 

[jazzjunkie]

Ah thats a shame i liked the simplicity and basicness of clay casting!

 

[qst42know]

There's no reason you couldn't make yourself a ring. :mrgreen:

The technique is just not well suited for bullion.

 

[jazzjunkie]

So what about calculating the force required to coldform silver... it seems to be simply the yield strength x area?

 

[qst42know]

Plug your numbers into this calculator and see what you come up with. According to the text coining force is ten times the air bending force.

http://www.pacific-press.com/bend-force-calculator-b.php