formula needed for weighing a wax model

[George]

I have a small wax model that is going to be cast into a silver pendant.  It weighs 1.8 grams.  So I have to find out how much silver to use.  I think I multiply by 11(for silver) but from there I am kinda lost.  I know I have to get Penny Weight somehow.  Can someone help?

[Russ Nobbs]

Accrding to Jeweler's Resource by Bruce Knuth you can multiply the weight of your model by 10.4 for sterling silver.  That will give you the approx weight in silver of the finished item.

Casting requires more silver than just the weight of the item. You need to fill the sprue and a solid button at the end of the sprue. To find how much silver you need for casting Knuth says to weigh the wax model with the sprue and multiply by 10.4. Add about 20% to that figure for the button on the end of the sprue.

Needless to say, R&T carries Jeweler's Resource. It's a handy book.

[Polly]

It is a handy book.  It has charts for just about every jewelry-related thing you might want: wire gauge charts, birthstone lists, basic geometry equations such as are, etc.
Book, Jeweler's Resource: A Reference of Gems, Metals, Formulas and Terminology for Jewelers

--Polly

[Russ Nobbs]

George has some other questions sent by PM. To allow other casters to reply and to benefit everyone I'm going to bring them back here to the public forum:
 

QUOTE

Russ:  Thanks for the info.  To make sure that I have it correctly:  I multiply the weight in grams of the wax model by 10.4 and that is the weight in grams of silver that I need.  So in my case it is 1.8 times 10.4 =18.72 grams.  Another question I have is---is there a possibility that a person can use too much silver or will it just create a dome at the sprue button?  

Also, it seems that it would make sense to fasten the wax model as close to the end of the sprue as possible  where the metal is to be poured.  My reasoning is that the longer the metal has to travel in the cast the cooler it would become and therefore the less chance of having a successful casting.  So if that is true then I would fasten the wax model at the bottom of the sprue--near the sprue base so when the wax is burned out and I turn the cast upside down to pour the metal the burn out part will then be at the top.  Thanks for any advice.                    
                                                    George

[Russ Nobbs]

That's close, George,

They suggest spruing it up first, then weighing it. You don't have the sprue added yet so you don't have the full wax model weight.  
The formula is ( (model and sprue weight) X10.4 ) X 1.2  
The 1.2 factor allows for the button.

Your model is 1.8 grams by itself. Let's assume your piece with the sprue is 2.4.
Your forumla will  be (2.4 grams X 10.4) X 1.2 or 24.96 X 1.2 or 29.95 grams . Round that to 30 grams.

Length of sprue. I can't answer. Maybe Kurt or another will jump in. My experience was with a long sprue so the model and the sprue filled up the length of a rubber mold. When doing one off items that's not an issue. Your suggestion of making the shortest distance for the metal to flow has merit. But why was I shown a longer sprue?  There may be a reason.

Too much metal? That can be a problem as it can overflow the flask splattering metal all over the walls of the casiting machine. (You DO have a cylinder/container  built around the casting machine, don't you? You don't want hot metal splattering you!)

[George]

Russ:  I am not using a centrifugal casting machine but am pouring it.

[Metalman]

So George, you will be using a vacumn cast or a steam cast method?

[George]

(Metalman @ Jun. 06 2006,12:11)

QUOTE

So George, you will be using a vacumn cast or a steam cast method?

I will melt silver with a torch in a crucible and pour into lost wax cast made of investment.

[Metalman]

George,
generally when casting, some pressure or pull is needed to get the molten metal into all of the mold details and textures.
3 low tech ways to do that are:
Steam casting, sling casting and vacumn casting. The centrifugal machines do what the sling method does - spin the mold with the molten metal around to force the metal into the details.
If you set up a table with small holes in it and attach a shop vac underneath and some gasket material on top you can set your hot mold on top of the gasket, flick on the vacumn and pour the metal immediately. The vacumn can suck through the porousity of the investment - off you go.
3rd is the steam casting. You can make a newpaper pad and put it inside of a jar lid with a handle. Soak it in water - then - pull your hot mold out of the kiln, pour your molten metal and press the newspaper pad lid down on the mold . Water vaporizes and steam casting occurs. These methods are all detailed in Tim McCreight's Book 'The Complete Metalsmith' pgs 95to 98
[R&T#62-010]
You may find , with some searching and reading - all of these methods on the internet.
Hope this helps

[George]

(Metalman @ Jun. 07 2006,14:33)

QUOTE

George,
generally when casting, some pressure or pull is needed to get the molten metal into all of the mold details and textures.
3 low tech ways to do that are:
Steam casting, sling casting and vacumn casting. The centrifugal machines do what the sling method does - spin the mold with the molten metal around to force the metal into the details.
If you set up a table with small holes in it and attach a shop vac underneath and some gasket material on top you can set your hot mold on top of the gasket, flick on the vacumn and pour the metal immediately. The vacumn can suck through the porousity of the investment - off you go.
3rd is the steam casting. You can make a newpaper pad and put it inside of a jar lid with a handle. Soak it in water - then - pull your hot mold out of the kiln, pour your molten metal and press the newspaper pad lid down on the mold . Water vaporizes and steam casting occurs. These methods are all detailed in Tim McCreight's Book 'The Complete Metalsmith' pgs 95to 98
[R&T#62-010]
You may find , with some searching and reading - all of these methods on the internet.
Hope this helps

Metalman:  I appreciate your advice.  What I had planned to do to get around the pull needed to get the metal into the 'lost wax cast' is to use a very short tree sprue and then use many 8 guage sprue waxes going from the sprue tree to the wax model.  I thought that I might cut some 8 guage sprue waxes in half longitudanally so I can get many small channels going to every area of the wax model---which would be many channels for metal to flow from the sprue button to the 'lost wax cast' area.  What do you think?

[Metalman]

http://www.ganoksin.com/
This is a great link for additional metalsmithing info.
There is a short discussion about making a burn out kiln out of a hotplate and a flower pot if you are interested.
Re: Sprues - size and length are related to several considerations
1] You want to plan in some room so you can get in and cut the sprues away with out damaging the cast
2] You need to balance the size of your excees button and you cast. You want the cast to draw metal from the button as it cools but not the reverse - this would give you a hollowed cast.
3] So - short and thick is good but with some caution. Length and diameter help balance the draw of the metal.
NOW - the surface detail and fineness are a consideration for a 'gravity' cast. Also the 'open work' aspect of the cast - meaning  whether it has lots of slender channel -like aspect to the design. This sounds like a fairly sizable cast, If you are going to do a number of them - May be just do a test and see how it goes. You can always remelt your silver and cast again. Sterling will tolerate about 10 remelts if my memory serves.
Last year, I took a workshop called Low Cost Casting. The steam method worked great and was cheap and easy. The low tech equip for this method and for the sling method is illustrated in the McCreight book.

[George]

Metalman:  I looked for the burn out kiln at the ganoksin site (There is a short discussion about making a burn out kiln out of a hotplate and a flower pot if you are interested.) but couldn't find it.  Do you have a specific address?  It sounds like something that will be helpful to me.   Thanks.
                                                    George