Can NOT get my silver molds hot enough

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08-09-2017, 02:27 PM
BurningKrome

Can NOT get my silver molds hot enough

Hello All,

I'm playing with silver casting, and I CAN NOT get my silver to flow all the way through the mold. I don’t have access to a kiln, and won’t for a while…so I’ve been using torches for the silver, and a Multigas camp stove to heat the mold.

For the campstove campstove, I’m using Rothernberger Multigas (70% propane, 30% butane) which can get to (measured) 700°C.

For the mold:
Mouldmaster 5 Kg Stonecast. The sprue is an approximately ½ tall wax cone, melted out in advance. I do put air vents between the wax cavity and the exterior of the mold using pins and nails when I make the mold (which are then pulled out after hardening). I know these vents remain patent, since heavy smoke billows out of them during the pour.

I’m melting the silver in an open-air crucible using a Rothenberger Superfire 2-Turbo Brazing Torch and MAPP gas (methyl Acetylene/Propane Gas Mix)

The process…
I put the mold, upside down, on the Multigas campstove (so that the pour hole is facing the flame directly) and then start melting the silver in the crucible. When the silver is really rolling (7-10 mins), I flip the mold over and bring the crucible’s pour to the mouth of the mold, and let the flame cover both the crucible and the mold for 15 seconds or so…and then briskly pour.

BUT, the silver never makes it through the entire mold. The best I’ve gotten so far is about 2/3rds of a ring.

I’ve tried putting the ring sideways instead of vertical, so the silver has to flow a shorter distance (no good).

I’ve tried two different kinds of oils in the mold first, including cooking oil and a high-temp (400 °C) graphite infused machine oil (the cooking oil worked best actually :D )…but still no good.

I have successfully done delft clay casting with silver, but this ring is too asymmetrical to do a standard delft clay casting (where the original is pulled out of the clay before casting). I have heard of using delft clay and wax…but the wax would have to stay in during the cast. Since delft clay and plaster have similar thermal conductivities, I’m not sure why one would work and not the other.

I have also thought of mixing a metal powder (zinc, copper, or aluminum...anything with a higher melting temp than silver) with the plaster (SiO₂ NOT CaSO₄…I don’t want to make thermite :D ) so that the cast can retain heat better, but I havent tried it yet. I did make a mold out of Stonecast plaster (SiO₂) and Aluminum powder…and it was so hard I couldn’t crack it (so I never tried casting in it).

Thanks for any advice! Again, my current limitation is I don’t have access to a kiln.
08-09-2017, 02:49 PM
Dennis

You say that you are using plaster, but you should be using casting investment, and it would be a mistake to contaminate it in any way.

Unless you are using several enormous sprues, it is unlikely that you will succeed by gravity alone. You will also need a casting machine, but there are some bargains to be had online.

However, you might investigate cuttle fish casting which is more low tech. Dennis.
08-09-2017, 03:07 PM
ps_bond

http://www.myheap.com/casting-moldin...html?showall=1
Definitely switch to a commercial investment - a 25kg sack is a pest, but it's a better bet than any non-investment plaster.
FWIW, Delft clay casting is entirely gravity fed (but yes, enormous sprues)
08-09-2017, 03:49 PM
josef1

Just for reference I cast silver with the flask at approx 620 deg C. What are you trying to cast
08-09-2017, 05:55 PM
BurningKrome

Hi Thanks for the comments.

So, for clarification, I am using a silver investment for casting (Stonecast). Its a Silicon Dioxide (SiO2) "plaster"...but its not really plaster. Should I be using a different brand?

@Josef1: Just small rings right now. But not rings that can be sand cast because the are too asymmetrical. But one of the problems seems to be; with the silver investment plaster I'm using, I can't seem to get the interior cavity above about 250 °C ... even using a 700°C flame blasting into it for 10 mins. The thermal conductivity on SiO2 (the investment) is really low (like 0.02) so it doesn't want to absorb the heat.

I haven't monitored the temperature of the silver while melting, but maybe I should. Do I need to perhaps make the silver hotter?
08-09-2017, 06:25 PM
josef1

I have never use stonecast so cant help sorry, if you use jewellery investment for best results you will need a kiln because you need to control the burn out this can take around 12 hours. I watched a chap on youtube call presstube he had some great casting videos, he had a gas powered melter but maybe you can make up a 'enclosure' out of fire bricks and heat the flask like that. I dont know if it would work though.

https://www.youtube.com/watch?v=07c7DgCCets
08-09-2017, 07:33 PM
handmadeblanks

Quote:

Originally Posted by BurningKrome

Hi Thanks for the comments.

So, for clarification, I am using a silver investment for casting (Stonecast). Its a Silicon Dioxide (SiO2) "plaster"...but its not really plaster. Should I be using a different brand?

@Josef1: Just small rings right now. But not rings that can be sand cast because the are too asymmetrical. But one of the problems seems to be; with the silver investment plaster I'm using, I can't seem to get the interior cavity above about 250 °C ... even using a 700°C flame blasting into it for 10 mins. The thermal conductivity on SiO2 (the investment) is really low (like 0.02) so it doesn't want to absorb the heat.

I haven't monitored the temperature of the silver while melting, but maybe I should. Do I need to perhaps make the silver hotter?

Hi,

Can I ask if you are using at least 4x the weight of the item to cast? i.e. If the item weighs 2g you need a minimum 8g to make a successful cast with a gravity casting method. I actually use much more: 15g to cast an item weighing 1.6g and I’ve never had a failed cast using that rule of thumb with delft clay but I’ve never used plaster. Maybe it's different. Sorry if I'm stating the obvious :">
08-09-2017, 07:46 PM
ps_bond

Quote:

Originally Posted by BurningKrome

So, for clarification, I am using a silver investment for casting (Stonecast).

Apologies - the precis I read didn't sound like a casting investment.
08-09-2017, 09:17 PM
Dennis

Betts Metals /Sutton Tools sell small quantities of investment; https://www.bettsmetalsales.com/p-In...ode=TOOLSACG43 Dennis.