Just wondering how to calculate the amount of metal needed when
casting from wooden, plastic models or any type of material for that
matter. I took a casting class a while back but lost my notes. Seems
like if the specific gravity of both the model and the casting metal
is known then there is an easy way to calculate the amount of metal
needed in order to make a cast. I’m doing small sand castings using
the Delft system if that should be of help in replying to this
message.

somebody is bound to give you a better answer than i, so i’ll keep
this short, but yes – what you need to know is the density per
volume of the material you’re casting in and the material you’re
casting into. there’s a great little iphone app called imakejewelry
which will transform silver to any karat of gold (and i think
platinum) for you – but a simple web search will do it as well.
metrically, if my highschool physics memory serves me correctly, a
milliliter is equivalent to one cubic centimeter’s worth of water. i
believe that has a specific gravity of one. (gotta love the ease of
the metric system). and then every other material has a specific
gravity that tells you how much one cubic centimeter of it would
weigh on earth. so let’s take an example. let’s say that your
material has a specific gravity of 2 and you are casting into silver
with a specific gravity of 10. let’s say that your sample weighs 1
gram and takes up 1/2 a cubic centimeter or 1/2 a millileter (hence
giving it a specific gravity of 2 as 1/(1/2) =2). then, your
equation to solve becomes 10=x/ (1/2) where 10 is the specific
gravity of the silver you’re casting into, 1/2 is the volume of the
piece, and x is the weight of the piece once cast — so in this
example, x grams. in reality, you’re going to know the weight of your
model, the specific gravity of your model’s material, ad then you’re
going to solve for its volume and plug in the result. hope this
helps!

Put the model into a glass and fill it to the brim with water. Take
out the model with a pair of tweezers. Put in casting grain or scrap
so that the water reaches the same level. Add enough for the sprue.
If you are in doubt add a sprue from a successful casting to the
model in the first step.

Quick and dirty… Since you did not specify if you wanted the
volume or weight of the metal for casting…

attach a pencil to the object (Call it your Sprue)

fill a glass to the rim with water (and leave the glass in the
sink as you will spill water)

Using the pencil push the object down into the water to about the
same depth it will be in the clay.

Assuming you have the metal that you will be using for the cast, add
it to the glass until the water is back up to the rim, add a bit more
for the button and you have the amount of metal needed.

If you are doing it so that you know how much casting grain to buy,
use a syringe so that you can keep track of the volume of water you
added and add water until the glass is full, this is equal to the
volume of the metal. Look up the specific gravity on line and do the
calculation to have the weight of the metal (pure water Specific
Gravity =1)

calculate the amount of metal needed when casting from wooden,
plastic models or any type of material for that matter.

Not so hard, Chris… First you need to understand that it’s only an
estimate - any casting process needs an excess of metal for spruing,
button, what have you. It doesn’t need to be exact, just good
enough.

There are a limited number of things you are going to be casting
FROM - wood, plastic, metal - and it’s all about specific gravity.
Water has an SG of 1 by definition. In the casting business we
consider things that float in water to be 1, just like water. That
means waxes of all kinds and wood, too. If you get really into it,
you might have to adjust for things like styrofoam, which is
especially light weight, or a large piece of lignum vitae, which is
especially heavy. But for jewelry scale items made in wax, plastic
or common woods you are looking at tenth pennyweight differences per
piece, which are absorbed by the button. That’s why it’s justan
estimate…

For metals, you use the ratio of the SGs of the metals. SS is 10.4,
14kt is 13.4, 18kt is 15.5, platinum is 21.5 but we use 22. Brasses
run around 8.5. Offhand I don’t know steel(s), but that is easily
researched.

So: 22 (platinum)/10.4 (sterling) equals 2.12, which means a
platinum piece will weigh 2.12 times as much as the sterling piece.
Same thing with whatever else - if it’s tin, use the SG of tin and
do the ratio… Not so hard…

Just wondering how to calculate the amount of metal needed when
casting from wooden, plastic models or any type of material for
that matter.

The fast and dirty method is to use a straight sided clear container
(a small graduated cylinder is ideal) of water. Put your model in
(you may need something like chopsticks or the like to submerge it
if it’s a floating material). Mark the line the water comes up to.
Take the model out and add metal til you match that line. Weigh it
out and add your sprue and button amount (for future reference).

Not as elegant or precise as the calculation method, but it’s fast
and it works.

Years ago, I used to get my casting done by a San Diego firm called
“San Diego Casting Co.”, no longer in business. I asked them the
same question about how they calculated casting weight from a wax to
be cast. They told me they take the sprued wax and for silver,
multiply by 16, for 14K gold, multiply by 18, and for 18K, multiply
by 20.

Since I have been teaching for over 25 years, my concern was to
simplify the casting procedures in our studio, to eliminate
complicated math ( which people always screw up ), provide enough
metal for each cast, and make sure all vital was
included on each casting flask. Probably 20-some years ago I designed
a stick-on label, which each person casting a flask fills out and
attaches to his or her flask. That label has a space for name, date
to be cast, a box for the sprued wax weight, weight formulas for
each metal, and another box for the final casting weight. Those
formulas, mentioned before are these:

Sterling silver: sprued wax X 16
14K gold: sprued wax X 18
18K gold sprued wax X 20

These formulas produce a casting weight with button, are quite
simple, and have served my classes successfully for over 20 years.

accurate method without measuring, weighing, and calculating and
especially useful for casting plastic objects, insects, leaves is to
get a clear glass jar large enough to put your model into. Fill it
halfway or so, drop the object in. Mark the jar where the water is
displaced. Take the model out, and put in enough of the metal you are
going to cast to again displace the water to the mark. You should be
set to go!

I found my class notes which say to simply multiply the weight of
the model in grams or pennyweight by the specific gravity of the
metal to be cast while adding 5 to 8 grams or 3 to 5 pennyweights to
get a complete cast. I like this method because it is straight
forward and the model can be made out of wax, wood, plastic, etc. All
you need is the weight of the model and the specific gravity of the
metal you are using to make the cast.

What I did:

dwt = pennyweight
1.6 dwt = weight of model
10.5 = specific gravity of sterling silver
5 dwt = weight added to get complete cast
10.5 x 1.6 = 16.8 dwt
16.8 + 5 = 21.8 dwt
21.8 dwt = weight of sterling needed for a complete cast

Explanation:

Weighed my model (wood in this case) which came out to be 1.6 dwt
(pennyweights).

The metal I used for casting was sterling silver (specific gravity
of 10.5).

So, 10.5 x 1.6 = 16.8 (weight of silver in dwt)

I added 5 dwt to 16.8 to get a complete cast. I decided to add 5 dwt
because the center of the model was partially hollow.

And yes… worked perfectly with my delft sand casting kit.

If anyone wishes a great informative wax/metal website here it is.
It gives you the weights in oz., dwts, etc’s. It also computes the
Day Value of Platinum, Gold, Silver, and costs of each for each wax
form you need.