Silver Hardened Too Fast When Casting

Did you heat the cast up some before pouring?

I'm from Romania and I'm not sure I undersatnd what do you mean by "eat the cast up some"

The flask with the investment was at 450 Celsius and the melted silver at 1000 Celsius.

As you see at 450 °C silver is solid. I think that the mold was at a too low temperature and the quantity of silver so small that it was cooled by contact. The thermal condcutivity of the form was also too high and its capacity to absorb heat to high too. You should increase the preheating temperature and look at the stuuf you use for the form.

Use above diagram which is independent of the country you are in or language you use.

I used silver 925, which means that my alloy contained 7.5% copper. The investment powder is Eurovest from SRS: http://www.srs-ltd.co.uk/products_services/eurovest.html

When I placed the flask at 450 Celsius on the silicon rubber adaptor on my vacuum casting chamber, the silicon was almost burned, so I don't think it could stand a much higher temperature.

Also, I'm wondering what could be the cause that the silver hardens when pooring, on the crucible that is still red from heat?

Thank you very much for helping me.

Why are you using a vacuum cast process? This appears to be the trouble.

Let me explain what happened. You heated up to 1000 °C. If you look at the graph under the mentioned temperature your mixture is SOLID. Between your initial temp and the solidus limit you only have about 35°C. Your form is heated up to 450 °C i.e. 550°C under your initial temperature. Do not forget that silver has a VERY high thermal conductivity. When it comes in contact with the cold form the fluid silver losses a lot of heat and will transfer heat to the form. But silver has also a low specific heat accumulation capability so that it will lower its temperature and due to its conductivity the lower temperature will spread all over and make even in the crucible the silver solid! Even if the crucible is red hot it cannot transfer heat enough to the metal in order to maintain temperature high enough. It is a SIMPLE problem of heat transfer ! If you want to fill a form then the silver MUST stay fluid till the form is totally filled if not with or without vacuum you will get holes.

No, here's the trouble as stated in the first sentence of the OP, "I started a jewelry workshop and I have no experience in this field".

So, one method of keeping the silver fluid would be to heat it with the torch when pooring?

There is a start for everyone. I learned to carv wax and now I'm trying to learn how to cast. I hope I'll make it with some help. :)

See the section on "model making and spruing" here :http://www.hooverandstrong.com/blog/archives/28/

It sounds like you are having problems due to the sprue design causing a bottleneck effect exactly as described by Carrano. Sprue thickness, placement of the sprue at the heavier part of the casting, etc etc. are the design solutions.

And here are a few other resources discussing sprue design:

http://www.ganoksin.com/borisat/nenam/wgc-sprues.htm

http://www.pro-relia.com.cn/products_detail/&productId=12b408c9-fcdd-4086-93ee-43f23f348692&comp_stats=comp-FrontProducts_list01-1280391666246.html

http://en.wikipedia.org/wiki/Sprue_%28manufacturing%29

It looks like you found a few helpful people here.

Good Luck.

As a child I remember watching my uncle he was a silversmith this is the only qualification I have in the field but he used a centrifical system made the wax shape then cast a plaster form & the whole lot mounted in a wound up spring drive unit bolted to the floor. He then heated the form to temperature & the silver was melted & poured into a section at the end closest to the centre of the assembly holding the form then heated the whole lot again & then set it to spin, that forced the silver in to the form

I am not experianced in this but I would think the vacume moulding system would cool the silver as the air passed over the silver unless there was constant heating of the metal & form as the silver was poured into the form the air flow would account for the silver becoming solid in the crucible as well

As I said I am not experianced in this field just thought I would tell my story lol

This is a link so you could see how is my casting procedure. (only I use carved wax models and I don't steem dewax). http://www.youtube.com/watch?v=MvOKy3Msa4I&feature=related

Maybe the insufficient vacuum could be a problem in my casting.

I will study the sprueing procedure from the links that you gave me.

When i made my wifes gold wedding ring i had the same problem the first time. The next time i ensured i had good sprue design ( also called runners and risers ). I burnt the wax out in a kiln at a temperature of over 1200 c, this ensures no carbon is left. i then melted the gold in a desert spoon in which i had placed some borax first. The borax prevents the gold/silver from 'wetting' the metal spoon and remaining there instead of going down the hole.

I did leave the mould in the kiln while i melted the gold with an oxy torch and opened the kiln and poured the gold in straight away. Good gloves are essential, need i say. Try doing some without the vacuum first as it will help you to develop good technique.

have fun.

Jim

If the vac pump is weak, some people recommend a liner.

Don't forget to toggle the switch so the vacuum is through the table instead of under the bell jar. (? Just a thought..)

You will find a lot of experienced discussion about vacuum casting online. The weight of the metal and height of your pour could also be a factor but doesn't account for the bottleneck effect so much as the sprue size and placement. The issue of sprue design is discussed by several experts in the vacuum casting threads:

http://www.ganoksin.com/orchid/archive/199903/msg00069.htm

http://www.ganoksin.com/orchid/archive/199903/msg00070.htm

The toggle is switched to the Casting Chamber

The problem is that my brand new vacuum machine is not working properly and I'm waiting a response from the manufacturer. Maybe I'm not doing something as I should.

Al my machines are new. First time I used teh kiln I put some borax powder in the graphite crucible. This formed a layer on the bottom of the crucible. Maybe this layer should exist on the entire surface of the crucible?

Maybe the sprueing is not very good, but there is defenetly another problem consiering that a small part of the silver hardened on the crucilbe while pooring.

Pay attention to the advice from Nick Name. You can bring your casting temperature up to 1040 C if it is cooling too quickly to pour. As Nick pointed out, this is always an issue with silver. It cools very readily on contact with any material or medium at a lower temperature (including the air) and becomes solid.

In my own work, I've noticed that the air temperature has a significant effect on processes involving fusing or flowing the silver surface. A slightly cooler temperature in the room and suddenly your process doesn't work as expected. Bring the room temp up a few degrees and everything is done quickly and easily...

this is good advice.

Some good advice - but you are wrong about vacuum casting. It is a common practice in silver and goldsmithing, these days. There are videos of it online, discussion in professional forums, etc etc. You may not have seen it, but there is no doubt it's widely used.

Vacuum casting for precious metals is S.O.P. with most production shops. It can and will produce excellent results once you get comfortable with the process. I am 99.9% sure the problem stated in this thread has NOTHING to do with faulty equipment or improper sprues. You just can't pour cold metal! And silver particularly likes things hot.

I have a friend who is a commercial jewelry caster, he pours hundreds of ounces per week (thousands of parts). He melts about 200 ounces at once in a simple gas fired pot furnace. As many as 40 8" flasks fit in his burnout oven. He pulls a hot flask from the oven and places it into his home built vacuum unit. Then he pulls the hot crucible from the furnace with a pair of tongs held in one hand, holds a torch in his other hand to keep things hot and then pours his flask. Repeating this process as many as 40 times in a day is seriously brutal work! His reject rate is near zero. I might add that he also owns a $20K hi-tech vacuum induction furnace which he very rarely uses, preferring the simplicity and high production rate of the primitive system.

I freely admit that I have not done much jewelry work for the last 15 years. I am native american and have only done one off casting so no high production experience. I only do personal gifts now. It is hard tedious work if you want to make money at it. I do think that the vacuum casters are not using silicon orings for seals on the flasks. Silicon orings just won't take the kind of temps needed for this.

In this case the guy appears to not be a high production shop and with the tools he has said he has never will be. He needs to get into centrifugal casting for one flask at a time casting. Vacuuming out the bubbles in the refractory material around his wax is a good use for his vacuum tool. I suspect that, that is what is was designed for if it came with silicon orings.

Someone posted some good links to how to make sprues. that should solve any problem that may have been there. Melting out the wax is in my opinion necessary for safety and a good pour. I have seen hot wax cause blowouts a few times. Re using the wax is more cost effective than burning it for another good reason to melt it out.

safety first.

As one who knows nothing about the subject of casting metal, but has an opinion about everything, I think the OP has gotten some good advice from you and others here, finally.

You just might make a caster out of him yet.

Cheers.

Thank you all for the answers. The next casting will have the fallowing changes:

- the room temperature will be at least 20 degrees Celsius (at the latest castings was 14 C)

- I will heat the silver at 1050 Celsius (was 1000 Celsius)

- I will hold the torch over the crucible while pooring the silver then over the silver button on the flask for a few seconds

I hope all this will make a successfull casting. Thank you all again.

Let us know how it works out. Good luck.

I will let you know how it worked.

The "silicon seal" is a hi temp gasket to form an airtight seal against the flange of the casting flask. It is designed to survive the rigors of properly heated flasks. Too much borax flux can cause a lot of trouble when casting silver jewelry work. Remember we are talking about melting very small amounts of material, a few ounces at most, not pounds. You really don't want a visable puddle of the stuff floating on top of your melt or you risk having it end up inside you mold cavity. Trust me, a pinch will suffice. If you see a black skin flash on the surface , add a pinch more. Just enough so the surface stays shiny and bright. Speed and coordination are the key goals to you will need to master. Get everything set up and try some dry runs (without hot metal). Choreograph your movements. Drop the flask into the vacuum table at the last possible moment, if possible leave the vacuum off until you have the crucible ready to dump. Yes, you will feel like you need 3 hands. Pouring the hot melt needs to be accurate and without hesitation. You might consider setting up an ingot mold to practice pouring a perfect stream into a small target hole. If you can't pour a perfect ingot, chances are you will be wasting a lot of time and material on missed castings. None of this is particularly diffcult but the process is intimidating at first, like riding a bike. Practice practice practice and like I said before, remember to relax and breathe normally.

I thought of something else, Graphite crucibles should NOT be fluxed previous to adding/melting metal. This will cause problems if you do, the silver will tend to permanantly stick/fuse to the crucible walls which is not a good thing . When using this type of crucible preheat it first add your metal and only add a pinch of borax when things get near the melting point. Ceramic crucibles are a different story. These need to be broken in by thoroughly glazing the insides with borax. Not so much as to form a puddle, but the insides need to be shiny. While things are hot, pour out any excess borax. You can make a scraping tool from an old stainless steel knife or spoon to help with this task. This breaking in is a bit of trouble but you only need to do it once. Personally I always preferred using this type of crucible.

So, I have to clean the graphite crucible from borax layer from the bottom.

How much do I have to heat the crucible before puting the methal in it?

Definitely not recommending a "puddle". The quantity she will need for the size crucible she is using (about what I used for vacuum testing for gas porosity) probably needs less than a thimble full. I probably used about a 1/4 to a 1/2 cup on a 300# melt of Chrome-Copper to simply produce a 'sheen"on the top, almost looked like a piece of saran wrap floating on the top. The reducing flame of a gas torch over the top surface is strongly recommended for silver also.

Silicon rubber generally has an upper limit of around 500 to 700 degrees F tops depending on the compounding used as I remember. Even the casters of white metals in silicon rubber molds use centrifugal vacuum casting to lengthen the life of the molds. Depending on the amount of vacuum needed the Fiber-fax can provide some amount of sealing depending on the weight of the flask or the clamping method if one is used. The limit of the silicone rubber seems to be too low to keep the investment at a high enough temperature for the thin cross sections probably involved here. There are Silicone Rubber greases available like a Dow Corning No. 7 that I used to use on shell core boxes. That I believe was rated for about 700 F. I used it to seal my vacuum gas tester.

Again not a big believer in vacuum tables. Heard them touted as a means to put more head on your pour and to remove air from the mold. Most refractories aren't permeable enough to allow air to be sucked through their walls. Most vacuum tables were simply meant to cool the plaster investment to keep it from disintegrating or giving off the water binder into the metal. Head is more a function of the height of your cope and the metal density. There is an investment caster in New England who actually uses a vacuum to fill the mold from the bottom up with the metal being sucked up into the investment and filling the cavities. Produces very high quality investment castings.

If the sprue is too small that also could be a problem since the cross section area of the sprue could be allowing too much contact with the investment walls. Investment material acts as a chill medium to the liquid being poured if its temperature is too low. I have seen many investment trees that have very large center sprue's which of course also have to act as the riser so it necessitates the larger cross sectional area than we required in sand casting.

You should be able to remove most of the borax from inside the crucible and return it to good service. To do this make yourself a hooked shaped scraper from an old stainless table knife. Use one with a rounded tip, heat it red with your torch and shape the end into a"J" shape. Now heat the crucible with your torch until the borax becomes fluid. Pour off or scrape as much of the borax as possible. This is also a good way to get comfortable with handling dangerously hot objects. Worst case; you will need a new crucible and you learned a valuable lesson or two. You can get the crucible hot 8-900c in your kiln before adding metal. This way the silver will melt even quciker. Some people measure out their silver and flux and wrap it in a piece of paper. This keeps everything nice & neat and adds a bit of carbon to aid in keeping oxygen away from the molten metal. If you are only pouring a few ounces at a time I recommend you melt it with the torch alone and not let it sit in the kiln. Using a hot crucible you should be able to bring 2-3 ounces of silver up to temp in less than 10 minutes. The quicker the better, just keep the torch moving in a circular manner and not to close to the surface of the melt. Pay attention to how the metal behaves and changes as it goes from solid to liquid. When you are able to slosh it around freely in the crucible, it will look/behave like mercury. NOW is the time to pour your mold or ingot or whatever. Forget the pyrometer! Keep a nice soft deep blue flame on top of the melt at all times. You should also keep a carbon stirring rod handy. this tool will let you remove excess flux or any dross that forms on top of the melt. A standard wooden pencil can serve this purpose but they burn up rather quickly. With time and patience you will learn to rely on your own senses and pouring hot silver will be become an intuitive act. Pyrometers and digital kiln minders are useful for monitoring burnout procedures but they will only distract and mislead you when trying to learn this essential skill.

Yes that's what I meant about the flux, just a sheen on top. Yeah the real function of the vacuum table for jewelry casting is to pull excess gases not to suck the metal into the mold. Come to think of it my commercial caster friend used a gasket something more like asbestos than silicone. The casting "flasks" in this case are perforated SS cylinders with a flange near the top. The casting table is simply a can with a hole at the top so the flange will seal against it. Jewelry investments are designed to be extremely porous and are near neutral in producing any off gases. They almost disolve instantly when quenched in water. Quite a bit different than investments used for larger work. I think Tallix Foundry was playing with that inverted vacuum method you mentioned? They must use one big ass pump! Like so many other processes, jewelry casting has many of it's own peculiarities. But the basic skills are still pretty much the same. For a beginner the first hurdle is getting comfortable with handling hot metal. I can't tell you how much metal hit the floor before I could nail a 1/2" dia sprue cup every time! Makes a person nervous with Silver @ $40.oz.

Hitchiner Mfg. is who I was referring to. I am familiar with Talix though. I met people from there at an Art Casting Symposium at the American Foundry Society back in the 90's when I was trying to start an Art Foundry. They cast a copy of DaVinci's Horse which was a very impressive undertaking back in 1999. I wouldn't be at all surprised if they attempted using vacuum on some of their work. When you a cast tumbleweed or thistle you know that pouring that without a misrun or coldshut is extremely difficult.

We did use asbestos sheet material up untill about the early 80's but due to the controversy I stopped utilizing asbestos and only used the ceramic cloth (Fiber-fax or equivalent) after that.

Had a colleague call me about removing the gas that they felt was causing porosity in yellow brass jewelry that his customer was making in a block investment. How do you get porosity with a metal that is 35-40% zinc since the partial pressure of the zinc keeps the H2 out. Turns out they were pouring the yellow brass at about 1875 F and the molds were breaking down releasing the water and causing the porosity. They were using casting plaster and not a good one evidentially for this work. To recover the plaster they would heat it up in an oven to drive off all the water and then re-use the plaster. Once they realized that the metal wasn't their problem they were able to work out a cooling solution for the molds.

I'm not sure of the exact formulation of jewelry investments but mostly they are gypsum/silica probably some alumina in there too. It is very lightweight and fragile after burnout. And it really does melt away like chalk when you quench a hot flask. Several/many years ago my commercial jewler friend asked me to do his production silver casting for him when he was laid up with a broken leg. After a month of 40 -60 flasks a week, neither love nor money would keep me at that job. I never wanted to set foot in another jewelry casting shop again. One of the most disgusting jobs was mucking out the investment sludge from the quench tank, a weekly task. Stunk like sulphur! My training and expertise is as a metalsmith so hammers and direct fabrication are my preferred tools. However, this new rennaisance for youngsters pouring iron in small homebuilt cupolas is great to see. Every once in a while i don leathers and fart around with it just for fun. Still, I'd never trade in my forge and hammers. -G

As part of the symposium we visited the Univ of Chicago Art Foundry. They had pictures of the day that they built a cupola outside and made iron and poured it for art out there in the open with people staring and gawking oblivious of the fact that cupolas have been know to explode from improper charging and people could get killed or maimed. Evidently the Chicago Fire dept got wind of it and told them "Never Again".

I am used to the heavy duty colloidal based investment where you need either a water canon(hydraulic jet) or hammer and chisel to break it apart after casting. Made some lovely parts with it though.

The sulphur smell might have been due to the gypsum. I think that sulphur is a by-product from gypsum plants so it probably is found in conjunction with the gypsum lodes.

Smithing is an old and proud profession. When you look in the phone book and see all the listings for Smith it does tell you that once were an awful of blacksmiths at one time. Probably went down hill after the introduction of gunpowder in Europe since the suit of steel armor did not hold up well against a little lead ball.

I'm so happy to let you know that I had a successful casting! Now I have a perfect wedding ring!

Here is what I did differently:

- room temperature: 24 Celsius

- silver temperature when casting: 1040 Celsius

- vacuum casting chamber functional

Thank you all for the advices, you've been a great help.

Congratulations! Best of luck with your future work.