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MEASURING & USING SPECIFIC GRAVITIES OF LEAD TIN ALLOYS By Wayne McLerran |

zeroing out the tare weight.

2. Suspend the bullet on the hook using sewing thread or thin monofilament

line. If everything is working correctly; the bullet should weigh the same as when

it’s placed directly on the scale platen.

3. The bullet is then suspended in water by simply raising the cup of water

under the bullet until it’s fully submerged. Make a mental note of the “wet

weight” reading while holding the cup steady and ensuring the bullet does not

contact the inside of the cup and nothing touches the fixture.

4. Using the dry and wet weights, determine the SG of the bullet alloy.

Formula B applies for this procedure. Using a dry weight of 427 grains and a

wet weight of 389.3 grains, SG = 427.1 ÷ (427.1 – 389.3) = 427.1 ÷ 37.8 =

11.2989; rounded to 11.30.

of very small or lightweight objects with a high resolution scale would benefit

from the increased stability when using the wire fixture and water cup platform,

but there’s no obvious advantage to using the technique for bullet casting and

lead alloy applications. Therefore the simpler procedure displayed in Figure 2

is the preferred technique.

batch. After tossing out the high and low values the remaining 8 were averaged.

The alloy I had mixed turned out to have a SG of 11.09, which equates to 22.8:1

with 4.2% tin, slightly off the goal of 20:1. The alloy that was reported to be

25:1 had a SG of 11.30, equating to a lead/tin ratio of 136:1 and containing only

0.73% tin. Hence, my friends alloy was way off the mark of 25:1. Using the

correct SG values and 427 grains for the 136:1 alloy bullet, the 22.8:1 alloy

should cast a bullet of 427 (11.09 ÷ 11.30) = 419.06 grains; close enough for

government work to the 420 grains I had posted on the forum.

that I highly recommend. It offers several features including the ability to

compute the percentage of lead and tin in the alloy based on SGs. For more

details on the calculator go to http://tmtpages.com/Alloy/alloycalc.htm. I used it

to quickly derive the lead/tin ratios from the measured SGs.

For another example, let’s determine what a bullet will weigh if the same mould

is used but the alloy is changed. So what will a 545 grain bullet cast from 20:1

alloy weigh if cast with 30:1 alloy? First calculate the SG of the alloys using

11.345 for the SG of lead and 7.337 for the SG of Tin. By the way, it’s a

common misconception that SGs of an alloy can be accurately calculated by

multiplying the weight or units of the alloy elements by their densities or SGs;

add the results and divide by the total weight or units. The correct formula is

more complex and is based on the reciprocals of the weights or units and

densities or SGs. For example:

SG of 20:1 = 1 ÷ [20 ÷ (21 x 11.345) + 1 ÷ (21 x 7.337)] = 11.0574

SG of 30:1 = 1 ÷ [30 ÷ (31 x 11.345) + 1 ÷ (31 x 7.337)] = 11.1485

slightly but will be well within the accuracy required for bullet casting. And

when solving the formulas above, remember to follow the mathematical “Order

of Operations” hierarchy, i.e. do things in parentheses first, multiply or divide

before adding or subtracting and always go from left to right.

Knowing that a bullet cast with 30:1 will weigh more than one cast with 20:1, the

next step is to multiply the bullet weight by the correct ratio of the SGs.

Therefore 545 (11.1485 ÷ 11.0574) = 549.49. Hence a 545 grain 20:1 alloy

bullet will weigh approximately 549.5 grains if cast with 30:1 alloy from the

same mould and under the same casting conditions.

describe lead/tin alloys. Some shooters use all the formats interchangeably

which is incorrect. Depending on the format used, there is a difference in the

alloy mix although it’s not large. The following examples should help clarify the

differences.

20/1, 20-1, 1/20, 1-20 or 1 in 20 implies 95.0% lead, 5.0% tin

19 units of lead + 1 unit of tin = 20 units of alloy

9.5 units of lead + 0.5 units of tin = 10 units of alloy

30/1, 30-1, 1/30, 1-30 or 1 in 30 implies 96.67% lead, 3.33% tin

29 units of lead + 1 unit of tin = 30 units of alloy

14.5 units of lead + 0.5 units of tin = 15 units of alloy

1:30, 30:1 or 30 to 1 implies 96.77% lead, 3.23% tin

30 units of lead + 1 unit of tin = 31 units of alloy

15 units of lead + 0.5 units of tin = 15.5 units of alloy

1:40, 40:1 or 40 to 1 implies 97.56% lead, 2.44% tin

40 units of lead + 1 unit of tin = 41 units of alloy

20 units of lead + 0.5 units of tin = 20.5 units of alloy

unseen voids in bullets have a direct affect on measurements of specific gravity

(SG). As noted earlier, Archimedes’ principal, which is the bases for SG

measurements, states that “Any object, wholly or partially immersed in a fluid, is

buoyed up by a force equal to the weight of the fluid displaced by the object”.

Therefore, since a bullet will displace the same volume of water regardless of the

size of internal voids, SG measurements are inversely proportional to the size of

the void. I.e. when using the air versus water weight technique, the SG of a bullet

with a large void will be lower than the SG of an identical bullet with a small

void. Hence, the large void bullet measurement will imply a smaller lead/tin

ratio. E.g. using a 530gr bullet cast with 20:1 alloy, if another bullet from the

same batch weighs 529gr due to a 1.0gr void, the SG of the 529gr bullet will

suggest the alloy ratio is 18.6:1. Therefore, when measuring SG to determine the

lead/tin ratio of an alloy, it’s wise to use bullets that fall within the upper end of

the weight spread, indicating minimum voids.

Wishing you great shooting,

Wayne

Updated 3/24/17

**The original article was published in the Summer 2014 edition (Issue #86) of **

the Black Powder Cartridge magazine and is posted with permission from

SPG, Inc. Please read the update information at the bottom of the article

prior to using this technique to determine the lead/tin ratio of your bullets

and alloy.

But before getting into the article and as a reference, listed below are the specific

gravities (SG) values of several well known bullet casting alloys. If you don’t

have access to an alloy calculator such as the one referenced in the article, once

the SG is measured you should be able to make a rough estimate of the lead/tin

percentage from the following values.

SG of pure lead is 11.3450

SG of 30:1 (lead/tin) is 11.1485

SG of 25:1 (lead/tin) is 11.1115

SG of 20:1 (lead/tin) is 11.0574

SG of 16:1 (lead/tin) is 10.9918

SG of pure tin is 7.337

**Note** - If there are elements in the alloy other than lead or tin the above SGs do

not apply. Therefore the values cannot be used for wheelweight alloys that

contain antimony, arsenic or other stuff. For alloys containing only lead, tin &

antimony here’s a link to an article containing an excellent chart of SGs: http:

//www.castpics.net/subsite2/Classics/Determining%20Alloy%20composition.pdf

By the way, although I used a digital scale for the article, with a little ingenuity a

beam balance scale will also work

****************************************************************

You may be wondering what lead alloys and Specific Gravity have in common

and expecting you to remember back to your high school or college general

physics or chemistry class is probably a stretch, especially for us older

“codgers”. So allow me to refresh your memory. Specific Gravity (SG), also

known as Relative Density, is the ratio of the density of any substance to the

density of a standard substance, water being the standard for liquids and solids.

To be precise, the SG of a solid or liquid is usually measured at a temperature

of 20°C and compared to the density of distilled water at 4°C. Therefore a

substance with a SG less than 1 will float on water, and will sink if the SG is

greater than 1. We have the Greek mathematician Archimedes to thank for

discovering SG around 212 B.C. So what does SG has to do with lead alloys

and bullet casting?

In many cases, using SG, the identity of an unknown element or simple alloy

components can be determined. Examples are alloys of lead and tin. I was

reintroduced to SG recently after posting on a well known BPCR forum that a

.40 caliber bullet cast from 20:1 (lead/tin) alloy weighed 420 grains and the same

bullet weighed 427 grains when cast from 25:1 alloy. Reading my post, an astute

shooter and experimenter with an engineering background took exception to the

alloy mix versus bullet weights and provided a convincing argument, using a

simple SG formula, that the increase in weight should be much smaller, only 1.35

grains to be precise.

For some time I’d considered purchasing or making a SG measuring setup to

confirm the purity of the lead and tin purchased and the mix percentage of

lead/tin alloys. Until recently all my bullet alloys were mixed from reportedly

pure lead and tin, therefore a tester was not essential. Now the contents of two

alloys were in question and SGs were required in order to determine the actual

lead/tin concentrations. Having mixed one batch from pure lead and tin I was

confident it was close to 20:1. The other batch, reported to be 25:1 but now in

question, was from a friend. To determine the actual mix a SG measuring setup

was needed.

__SG Measuring Techniques__

Archimedes’ principal states that “*Any object, wholly or partially immersed in a *

fluid, is buoyed up by a force equal to the weight of the fluid displaced by the

object”. Therefore, depending on the measurement technique detailed here, two

formulas are applicable. Formula A: SG = weight of object in air ÷ weight of

object in water. Formula B: SG = weight of object in air ÷ (weight of object in

air - weight of object in water). To simplify the formulas the weight of the object

in air will be referred to as the “dry weight” and the weight of the object in water

will be the “wet weight”. Hence, to obtain the necessary values for the

calculations, the object is 1st weighed in air then weighed again while suspended

in water, referred to as hydrostatic weighing. In theory this seemed easy enough

but a digital scale is required. Ideally the scale should be capable of a resolution

of 0.01 grains, but 0.1 grains is sufficient if a careful measurement technique is

used as explained later.

SG measuring setups are available from numerous laboratory equipment

manufacturers. An example of one setup is shown in Figure 1. But even

simpler solutions will suffice for the bullet caster. After additional research I

realized the platform shown in Figure 1 to hold the cup of water is not absolutely

necessary. In fact a reasonably accurate measurement can be made without the

wire frame fixture to hold the bullet or object.

the Black Powder Cartridge magazine and is posted with permission from

SPG, Inc. Please read the update information at the bottom of the article

prior to using this technique to determine the lead/tin ratio of your bullets

and alloy.

gravities (SG) values of several well known bullet casting alloys. If you don’t

have access to an alloy calculator such as the one referenced in the article, once

the SG is measured you should be able to make a rough estimate of the lead/tin

percentage from the following values.

SG of pure lead is 11.3450

SG of 30:1 (lead/tin) is 11.1485

SG of 25:1 (lead/tin) is 11.1115

SG of 20:1 (lead/tin) is 11.0574

SG of 16:1 (lead/tin) is 10.9918

SG of pure tin is 7.337

not apply. Therefore the values cannot be used for wheelweight alloys that

contain antimony, arsenic or other stuff. For alloys containing only lead, tin &

antimony here’s a link to an article containing an excellent chart of SGs: http:

//www.castpics.net/subsite2/Classics/Determining%20Alloy%20composition.pdf

By the way, although I used a digital scale for the article, with a little ingenuity a

beam balance scale will also work

****************************************************************

You may be wondering what lead alloys and Specific Gravity have in common

and expecting you to remember back to your high school or college general

physics or chemistry class is probably a stretch, especially for us older

“codgers”. So allow me to refresh your memory. Specific Gravity (SG), also

known as Relative Density, is the ratio of the density of any substance to the

density of a standard substance, water being the standard for liquids and solids.

To be precise, the SG of a solid or liquid is usually measured at a temperature

of 20°C and compared to the density of distilled water at 4°C. Therefore a

substance with a SG less than 1 will float on water, and will sink if the SG is

greater than 1. We have the Greek mathematician Archimedes to thank for

discovering SG around 212 B.C. So what does SG has to do with lead alloys

and bullet casting?

In many cases, using SG, the identity of an unknown element or simple alloy

components can be determined. Examples are alloys of lead and tin. I was

reintroduced to SG recently after posting on a well known BPCR forum that a

.40 caliber bullet cast from 20:1 (lead/tin) alloy weighed 420 grains and the same

bullet weighed 427 grains when cast from 25:1 alloy. Reading my post, an astute

shooter and experimenter with an engineering background took exception to the

alloy mix versus bullet weights and provided a convincing argument, using a

simple SG formula, that the increase in weight should be much smaller, only 1.35

grains to be precise.

For some time I’d considered purchasing or making a SG measuring setup to

confirm the purity of the lead and tin purchased and the mix percentage of

lead/tin alloys. Until recently all my bullet alloys were mixed from reportedly

pure lead and tin, therefore a tester was not essential. Now the contents of two

alloys were in question and SGs were required in order to determine the actual

lead/tin concentrations. Having mixed one batch from pure lead and tin I was

confident it was close to 20:1. The other batch, reported to be 25:1 but now in

question, was from a friend. To determine the actual mix a SG measuring setup

was needed.

fluid, is buoyed up by a force equal to the weight of the fluid displaced by the

object”.

formulas are applicable. Formula A: SG = weight of object in air ÷ weight of

object in water. Formula B: SG = weight of object in air ÷ (weight of object in

air - weight of object in water). To simplify the formulas the weight of the object

in air will be referred to as the “dry weight” and the weight of the object in water

will be the “wet weight”. Hence, to obtain the necessary values for the

calculations, the object is 1st weighed in air then weighed again while suspended

in water, referred to as hydrostatic weighing. In theory this seemed easy enough

but a digital scale is required. Ideally the scale should be capable of a resolution

of 0.01 grains, but 0.1 grains is sufficient if a careful measurement technique is

used as explained later.

SG measuring setups are available from numerous laboratory equipment

manufacturers. An example of one setup is shown in Figure 1. But even

simpler solutions will suffice for the bullet caster. After additional research I

realized the platform shown in Figure 1 to hold the cup of water is not absolutely

necessary. In fact a reasonably accurate measurement can be made without the

wire frame fixture to hold the bullet or object.

scale platen. The container should be as lightweight as possible but large

enough to hold a submerged bullet without the bullet coming in contact with the

container. I used a prescription pill container. Figure 2 illustrates the 3-step

measurement procedure.

1. Assuming the scale is warmed up and calibrated, place the bullet on the

platen. Measure and record the dry weight.

2. Place the container of water on the platen and zero out the tare weight.

**Note - **If the scale does not have a zeroing or tare weight feature, the

measurements can still be made but the scale must be capable of accurately

measuring the total weight of the container of water and bullet to an accuracy of

0.1 grains minimum. In this case the weight of the container of water must be

measured and subtracted from the wet measurements.

3. Using sewing thread or thin monofilament line, suspend the bullet fully

submerged. Note the “wet weight” reading while ensuring the bullet does not

contact the inside of the container.

4. Using the dry and wet weights, determine the SG of the bullet alloy.

Formula A applies in this case. Using a dry weight of 427 grains and a wet

weight of 37.8 grains, SG = 427 ÷ 37.8 = 11.2963; rounded to 11.30.

**Note -** Although the test sample and faucet water will be at room temperature,

the resulting accuracy of the procedure will be sufficient for our needs assuming

the scale has a resolution of at least 0.1 grains, several careful measurements are

made and the results averaged,. Out of curiosity I did try very cold distilled

water and there was no measurable difference within the resolution of the RCBS

scale.

__Constructing a Test Fixture__

Although the very simple procedure seemed to work fine, I wondered if the wire

fixture on the commercial setup shown in Figure 1 offered an advantage. Using

it as a model, the fixture displayed in Figure 3 was constructed. It had to be

sufficiently lightweight to allow zeroing out its “tare weight”. The fixture

consists of a small cork, four pieces of thin wire, a thin plastic disk from a

compact disk container and a lightweight plastic cap off an aerosol can. The cap

is slightly larger in diameter than the scale platen. It was cut down to the

thickness of the top of the platen and glued to the disk. Although not absolutely

necessary, the cap centers the fixture and keeps it from sliding off the platen.

The fixture is tall enough (8”) to insert a small handheld cup of water under the

bullet without spilling the water or touching anything. The space between the

main support wires is sufficient to allow entry of my hands while holding the

cup. The fixture weighs a little less than 400 grains.

platen. Measure and record the dry weight.

2. Place the container of water on the platen and zero out the tare weight.

measurements can still be made but the scale must be capable of accurately

measuring the total weight of the container of water and bullet to an accuracy of

0.1 grains minimum. In this case the weight of the container of water must be

measured and subtracted from the wet measurements.

3. Using sewing thread or thin monofilament line, suspend the bullet fully

submerged. Note the “wet weight” reading while ensuring the bullet does not

contact the inside of the container.

4. Using the dry and wet weights, determine the SG of the bullet alloy.

Formula A applies in this case. Using a dry weight of 427 grains and a wet

weight of 37.8 grains, SG = 427 ÷ 37.8 = 11.2963; rounded to 11.30.

the resulting accuracy of the procedure will be sufficient for our needs assuming

the scale has a resolution of at least 0.1 grains, several careful measurements are

made and the results averaged,. Out of curiosity I did try very cold distilled

water and there was no measurable difference within the resolution of the RCBS

scale.

fixture on the commercial setup shown in Figure 1 offered an advantage. Using

it as a model, the fixture displayed in Figure 3 was constructed. It had to be

sufficiently lightweight to allow zeroing out its “tare weight”. The fixture

consists of a small cork, four pieces of thin wire, a thin plastic disk from a

compact disk container and a lightweight plastic cap off an aerosol can. The cap

is slightly larger in diameter than the scale platen. It was cut down to the

thickness of the top of the platen and glued to the disk. Although not absolutely

necessary, the cap centers the fixture and keeps it from sliding off the platen.

The fixture is tall enough (8”) to insert a small handheld cup of water under the

bullet without spilling the water or touching anything. The space between the

main support wires is sufficient to allow entry of my hands while holding the

cup. The fixture weighs a little less than 400 grains.