# Calculating Ratios

#1

I keep running into this and never was real good at math
anyway…typically I’ll get some RTV stuff or investment ratios or
whatever and need the formula for figuring mixing ratios. Example 9
parts silicone to 1 part hardener. I’ll fill my mold with water for
volume, pour it into a paper cup and mark a line where the top level
of the water hits. I’ll pour in the silicone liquid to that point and
weight it. So say its 115 grams for instance, how do I figure that
tenth part based on the 115 gram weight? Dave

#2

Dave, In your example divide 115 by 9. That will give you the one
tenth part weight.

Jim

#3

Dave,
x = 100% weight
use 9 parts = 115 or .9x = 115gms.
x = 115/.9 = 100% weight
100%weight - .9 weight = 1part weight
as per example;
.9x = 115 gms
x = 115/.9 =127.778
127.778 - 115 = 12.778 gms equals 1 part

``````Test = 9 x 12.778 = 115.002 check!!
``````

Good Luck!

#4

Dave,

If you’re going to use 115 grams of silicone, that is nine parts of
the whole, so you would want to add enough to make the weight 10/9 of
that, that’s 9/9 plus 1/9 total, for ten parts. Divide the 115 by 9,
that’s 12.78 grams of hardener.

If you were wishing to make the total 115, you would divide by 10 to
find how much of it should be hardener, that’d be 11.5 grams, then
subtract that from the whole, calculating that 103.5 grams of silicone
liquid are required.

Hope this helps.

#5

If ‘9’ becomes 115, then ‘1’ must be a ninth. Divide 115 by 9.

115 grams silicone to 12.8 grams hardener total 127.8g 9 parts
silicone to 1 part hardener

Bri
B r i a n A d a m J e w e l l e r y E y e w e a r
@Brian_Adam1 ph/fx +64 9 817 6816 NEW ZEALAND

#6

Dear Dave

The answer is that you don’t. And you can’t using your method.

Silicone rubber does not have the same specific gravity (or density)
as water, so the volume of water you measured has no relation to the
weight of the same volume of rubber. Think of a pound of feathers vs.
a pound of carrots. Different volume entirely.

What you need is a scale. If the ratio called for is 9 to 1, then
you need 9 grams of rubber to 1 gram of catalyst. Or 18 to 2. Or 27 to
3 , etc.

How many grams do you need for that mold? There are formulas, but far
easier to is take an already finished mold a weigh it. Each mold won’t
be exactly the same, but it’ll be close. Yes, you may have mixed a
little too much on occasion, but what is the cost of a total
failure??
Michael Knight

F.E. Knight, Inc.
120 Constitution Blvd.
Franklin, MA 02038
United States of America
Voice: 508-520-1666
Fax: 508-520-2402

#7

If 115gm is .9 of the total weight, then
T.W. = 115 divided by .9 or 127.78. Therefore-
the weight of the hardener is 127.78 minus 115 or 12.78gms.

``Hope this helps     J.Z.Dule``

#8

Hi Dave. What I would do is:

1.Fill the mould with water, pour it into a paper cup, weigh it, mark
it, pour it out.

1. Divide the weight of the water by ten (10). Weigh out that amount
of water and pour it into your paper cup. Mark it, then pour out the
water.

3.Now pour hardener to the first mark, then fill to the top mark with

Will Estavillo, www.natureshop-gallery.com

#9
`````` I'll pour in the silicone liquid to that point and weight it. So
say its 115 grams for instance, how do I figure that tenth part
based on the 115 gram weight?
``````

Your problem here is you’re mixing apples and oranges - you can’t
figure the hardener by weight unless you’re sure that it’s the same
density( specific gravity) as the silicone - and that’s very
unlikely. You have to do it by volume - if you’ve got 200 milliliters
of silicone, add 20 ml of hardener - like that. The other way you can
do it is to figure the density of the silicone - say it’s 115 grams
for .5 liter - and compare that to a similar quantity of hardener -
say it’s 70 grams for .5 liter - then you divide the 70 gr. by the
115 gr. and you learn that hardener is about 61% as dense as
silicone, so to mix 10% hardener into 115 grams of silicone, you need
to multiply 115 x .10 to get 11.5 , which would be the weight to add
if hardener were as dense as silicone, then multiply that by .61 to
get 7.01 grams of hardener. DO NOT USE THESE NUMBERS - I just made
them up - you’ll have to do the actual weighing and measuring
yourself - or find a handy lab reference guide where someone has
already figured these things out. Good Luck.

Mike

#10

Also, most silicon “hold” for many days in the freezer if you end up
with more catalized rubber than currently needed. Won’t work for poly
sulphides, and most eurathanes as far as I know. Just a thought .

John Dach

#11
``````The answer is that you don't. And you can't using your method.
Silicone rubber does not have the same specific gravity (or density)
as water, so the  volume of water you measured has no relation to the
weight of the same volume of rubber. Think of a pound of feathers vs.
a pound of carrots. Different volume entirely.
``````

Not at all, Michael, Dave can use his method. He used water just to
to transfer the volume of his mold to a paper cup. He marked the cup.
Then he poured silicone into the cup up to the mark and weighed the
silicone, not the water.

He needs a simple method of calculating a 9:1 ratio when 9=115.
Everone’s come to the same conclusion so far: 115/9.

Bri

B r i a n A d a m J e w e l l e r y E y e w e a r
@Brian_Adam1 ph/fx +64 9 817 6816 NEW ZEALAND

#12

Hello, If Dave FILLS his mold with water and pours that into a paper
cup to be measured, the volume is at max. You cant add hardener, or
any thing else without overflowing the mold. So, that means his weight
of 115 grams is academic and leads to doing things the hard way. I am
assuming that he wants to find ratios by volume, (9:1 in this case
means 9 parts to one part by volume). As someone pointed out earlier
the specific gravities of hardener and silicone may be different and
might mean that 11.5 grams of HARDENER (1/10 total weight) will have a
volume more (or less) than necessary to fill the mold when added to
103.5 grams silicone. However, 11.5 grams of SILICONE will give you
the correct VOLUME of hardener to add to 103.5 grams of silicone to
exactly fill the mold. In a previous post I suggested how to do this.
Now lets see, two parts scotch, one part ice… Will Estavillo,
www.natureshop-gallery.com

#13

Lets look at this problem another way. We will make the assumption
that Dave wants to fill the mold exactly with a mixture of 9 parts of
silicone rubber and one part of hardener. That means that there is
going to be a total of ten equal parts. (9+1=10). Now, Dave fills the
mold with water to get the correct volume and pours it into a cup. For
the sake of this argument let’s say the paper cup is a skinny one
exactly 10 centimeters (or inches you choose) high and the water fills
it exactly to the top. There are 10 equal marks ruled on the cup. The
top mark is also the top of the cup. The bottom 9 sections are
reserved for silicone and the top section for hardener. Are you with
me? Now Dave pours out the water and starts to fill the graduated
paper cup with silicone rubber. Lets stop for a moment when Dave
reaches the 9th mark. What is the weight of the silicone up to the 9th
mark? Well, if he fills the cup all the way and weighs it, it is 115
grams. So 9/10 of 115 gm = 103.5gm. Good, we have the weight of the
silicone. But what about the hardener? We know it is going to fill the
top 1/10 portion of the cup. NOW, lets say Dave has three different
hardeners to choose from. One is a uranium/lead based solution that
weighs about …50 grams/ cm. Another has the same density as the
silicone and weighs 11.5 grams/cm. And a third solution is made from
Xenon and has a density of .1gm/cm. Wait a minute! YOU CAN’T GET THE
WEIGHT OF THE HARDENER FROM THE TOTAL WEIGHT OF SILICONE. You CAN get
the VOLUME of hardener; but, to determine the weight of any fluid that
fills that last 1/10 you need its density ( specific gravity). Dave,
unless you have the specific gravity of the hardener, you have to do
your calculations using volumes. By the way, if you divide 115 by 9
you get a ratio of 1:8, gotta work fast. Regards, Will Estavillo
www.natureshop-gallery.com

#14
``````Lets look at this problem another way. We will make the assumption
that Dave wants to fill the mold exactly with a mixture of 9 parts of
silicone rubber and one part of hardener.
``````

By weight r by volume? I was assuming by weight.

``````unless you have the specific gravity of the hardener, you have to do
your calculations using volumes. By the way, if you divide 115 by 9
you get a ratio of 1:8, gotta work fast.
``````

If you divide 9 by 1 you get a ninth. Add that to the 9 parts and
that’s 10 parts.

However I do see the flaw in my water in the mold system. I see what
you mean now. Quite a sticky problem, I guess. Maybe reading the
instructions would help?

Nice to nut this one out with y’all.

Bri

B r i a n A d a m J e w e l l e r y E y e w e a r
@Brian_Adam1 ph/fx +64 9 817 6816 NEW ZEALAND

#15
``````By the way, if you divide 115 by 9 you get a ratio of 1:8,
``````

Bri, Oops! I wasn’t very clear on this point. If you divide the total
weight by 9, you get 9 parts, One part is the hardener and 8 parts is
the silicone. Remember, the 115 grams is the total weight (and
volume). You cant add anything to it without overflowing the mold.
Regards, Will E.

#16

weight of the base material is 115 grams and this must be 0.90 of the
total weight, then:

Total Weight X 0.9 = weight of base material=115gms solving this
equation for T.W. you get T.W= 115gms divided by 0.9 or127.78 gms;
therefore Weight of accelerator = T.W. minus wgt of base or 127.78
-115 or12.78 gms 115/127.78=0.9 OR 9% AND 12.78/127.78=0.1 OR 1%

I hope this helps those who are not mathematically inclined. J.Z.
Dule

#17
``````Remember, the 115 grams is the total weight (and
volume). You cant add anything to it without overflowing the mold.
Regards, Will E.
``````

Hi there Will. I was thinking this too, but thought some of the
excess might stay stuck to the walls of the container (not
sufficiently mixed, probably) or just be left as waste, on the
assumption that too much is better than…

Is there just you and me left in this room? maybe we could go and
have a coffee now?

Cheers
Bri

B r i a n A d a m J e w e l l e r y E y e w e a r
@Brian_Adam1 ph/fx +64 9 817 6816 NEW ZEALAND