TexasMac's Web Site

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

fixture and 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.

measuring the SG 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.

made of each 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.

available 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

will vary 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.

used to 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

clear that 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 gravity (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.

#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.

the specific gravity (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

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.

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

the fluid displaced by the object”.

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.

on the 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.

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.

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.

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.

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.