Hardness Testing Lead – A series of TIPS from my recent Cast Lead Bullet Workshop

Hardness Testing Lead – A series of TIPS from my recent Cast Lead Bullet Workshop

I am getting a lot of inquiries from my Casting Class students regarding hardness testing. That raises the first, and most important question:

WHY is lead hardness important?

Simply, the faster a bullet gues through the barrel, the harder the alloy needs to be. That’s because of a concept known as OBTURATION. Obturation is the effect of the gas pressure in the case/cartridge during the burning of the powder. The gas pressure builds until it spikes just right, and then the bullet leaves the case mouth. That’s all very quick (POP!), but during that time, the back end of the cast lead bullet needs to mushroom or blossom from the gas pressure, sealing the back end of the bullet into the lands and the grooves. That prevents the hot plasma gases from leaking past the bullet, the primary cause of lead fouling. That is to say, lead skid marks. If the next bullet down the barrel also causes more lead fouling, each successive bullet will get nudged a bit (in the direction of the skid mark), and start really throwing the bullets off course. So, in order for this obturation to happen effectively in the very limited amount of time, the bullet has to be cast in an alloy hardness that facilitates that obturation. The slower the bullet, and consequently the gas pressure, the softer the alloy needs to be. The faster the bullet goes, and consequently the higher the pressure, the harder the alloy needs to be to achieve the same resulting obturation. The best way to attack the problem is from known experience with cast lead bullets. The following charts highlight some hardness points relative to muzzle velocities for guns, both pistol and rifle

 

 

 

 

 

Page 30 of Glen Fryxell’s Book ( Referenced Below) has an amazing discussion of lead alloy hardness, ingredients (typically Lead, Tin, and Antimony), and their uses. He also discusses the concept of OBTURATION is some detail. Especially starting on Pg 32. I will leave it to the reader to spend some time with Glen’s book.

 

Lead Hardness Testing

While there are several lead hardness testers on the market, some of them provide obscure hardness references. Only one that I know of provides a clear “BHN” Scale hardness result. Just exactly what is a “BHN” Scale Hardness result? BHN stands for Brinell Hardness Test. It was first suggested in 1900, and is one of the ways to measure hardness of a material (wood, brass, lead, aluminum, steel, etc.)

The WIKI for the Brinell hardness testing, the real subject of this article is here:
https://en.wikipedia.org/wiki/Brinell_scale

In a laboratory setting, a hardness test is commonly done using a calibrated machine/device which presses down on the test sample with a given force, and the amount that the point of the device presses/penetrates into the test sample is shown on a scale. When measuring hard metals, such as steel, a sharp point is used, and the hardness result is given on the “C” scale. Rockwell is one of the leading companies making these high end hardness testers.

These laboratory testers can cost as little as $1600, with the full digital ones selling for almost $6700. The laboratory units come with a full set of contact points suitable for a full range of metals to be tested. The two common hardness ranges tested on these scales are the C scale and the B scale. The B scale refers to softer alloys, and for those a 1/16″ hardened ball is used to penetrate the softer metal. That includes the lead alloys. Here’s the WIKI reference for the Rockwell Hardness Test, and the reference hardness scales that they produce:
https://en.wikipedia.org/wiki/Rockwell_scale

But, at $1600 for the least expensive ones, it’s a bit of overkill for the home caster of cast lead bullets. And, the “B” scale for Rockwell testers, while similar to Brinell Hardness, isn’t an exact same hardness number. We really want to spend our time using the BRINELL Hardness test, with a Resulting BHN number. A more cost effective scale would always make more sense for someone casting lead bullets, and needing some alloy hardness guidance so that the bullets obturate properly upon firing the cartridge.

Hardness testers for Bullet casters

I have one really good recommendation for a hardness tester that provides the most versatile testing of lead alloys. It can be used with cast bullets, ingots, slabs, etc. It gives a BHN hardness result, which can be directly referred to.

*My number one recommendation for hardness tester for bullet casters:

Awaiting permission from that manufacturer for using images and information for the tester that I like the most, and recommend.

There are other contenders for hardness testers appropriate for bullet casters. They include the Saeco Tester, the Lee Hardness Tester, and the Cabine Tree Tester.

The Saeco Tester has a limitation in that you can only check small samples, typically an actual cast lead bullet. And, the use of that tester requires visually aligning two lines, and determining from that what the hardness is. The Saeco scale of hardness does not directly correlate with the Brinell hardness numbers (BN). The Saeco tester costs just under $200.

 

The Lee tester uses an indenter system, and then a magnifying glass with a small chart of hash marks. You are supposed to visually determine the diameter of the indent in the lead using the magnifier and chart. It’s awkward to use, and gives non-consistent numbers. It is about $90.

 

 

The Cabine Tree tester uses a spring loaded indenter, and a rotating handle to activate the indenter. A dial indicator measures the amount the indenter pushes into the soft alloy. That depth can be converted into a hardness number from a chart they supply with the device. The Cabine Tree Tester costs about $125.

 

You may have secured your raw lead alloy from a number of places, plumbing fixtures, roofing flashing, range lead, wheel weights, linotype, etc. The starting hardness of the alloy typically needs to get adjusted. Lead alloys are commonly comprised of Lead (Pb), Tin (Sn), and Antimony (Sb). The presence of tin in the alloy is usually to assist with the flowing of the alloy, especially in tight places in your casting mold. It does little to harden the alloy.

Antimony is the most common element for hardening the lead alloys. As found, it is a crystalline “mettalloid”.

It is NOT suggested that anyone attempt to crush up elemental antimony, and try to get it to dissolve in your alloy. Any of the hard crystal that doesn’t get dissolved would be an abrasive, to be avoided in your barrel. Rather, the best way to get antimony into your alloy is to use other lead alloys, which are high in antimony (already dissolved.) Such additives include clip on wheel weights, linotype, and monotype.

Clip on wheel weights contain about 3-4% antimony, and are (as air cooled) about 10-11 BHN.  Linotype is about 12% antimony, and in the range of 22 on the BHN scale. Monotype is a little higher, with the antimony being about 19% and hardness in the range of 28 BHN.

If you have very soft lead to start with, you need to add one of those hardening alloys. If you have lead too hard for your current use, then add some pure lead (about 5 BHN) to get it right. You can approximate the amount of hardener or pure lead needed to get to a proper range. Or, there are a number of on-line calculators available to assist with getting it more exact.
Here’s an on-line spreadsheet calculator, free to download and use:
Free Spreadsheet Calculator

Here’s a link for some software that is economical to purchase, and does an amazing job of guiding your adjustments:

Cast Bullet Alloy Calculator Ver. 1.1.4 Download (approx $20)

From RotoMetals, a supplier of alloy bars, is this tip for hardening lead alloy:
For every 1% additional tin, Brinell hardness increases 0.3.
For every 1% additional antimony, Brinell hardness increases 0.9.
For a simple equation,
Brinell = 8.60 (Antimonial Lead) + ( 0.29 * Tin ) + ( 0.92 * Antimony )

Some additional reference details:
Description, Composition, Uses, Hardness, Est. Hardness
Pure Lead, (100% Lead), Minies, Round, Brinell 5, 8.6
Pure Tin, (100% Tin), Alloy, Brinell 7, –
Pure Antimony, (100% Antimony), Alloy, Brinell 50, –
Antimonial Lead, (5% Antimony, 95% Lead), Alloy, Brinell ???, 13.2
Super Hard Alloy, (30% Antimony, 70% Lead), Alloy, Brinell ???, 36.2
40 to 1 Alloy, (2.5% Tin, 97.5% Lead), Blackpowder, Brinell 8, 9.3
30 to 1 Alloy, (3% Tin, 97% Lead), Blackpowder Brinell 9, 9.5
25 to 1 Alloy, (4% Tin, 96% Lead), Blackpowder Brinell 9, 9.8
20 to 1 Alloy, (5% Tin, 95% Lead), Blackpowder Brinell 10, 10.1
Chilled Shot, (2% Antimony, 98% Lead), Shotgun, Brinell ???, 10.4
Magnum Shot, (5% Antimony, 95% Lead), Shotgun, Brinell ???, 13.2
Wheel Weight – Stick On, (0.5% Tin, 99.5% Lead), Minies, Round, Brinell 6, 8.7
Wheel Weight – Clip On, (0.5% Tin, 2% Antimony, 97.5% Lead), Pistol, Rifle, Brinell 12, 10.6
Lyman’s No. 2, (5% Tin, 5% Antimony, 90% Lead), Pistol, Rifle, Brinell 15, 14.7
Hardball Alloy, (2% Tin, 6% Antimony, 92% Lead), Pistol, Rifle, Brinell 16, 14.7
Linotype Alloy, (4% Tin, 12% Antimony, 84% Lead), Alloy, Brinell 19, 20.8
Monotype Alloy, (9% Tin, 19% Antimony, 72% Lead), Alloy Brinell 26, 28.7

Knowing what your lead alloy is to start with, knowing what it needs to be for success with cast lead bullets, and knowing what to add and how much of that to add is the key to success. Obturation is what makes it happen correctly.

Reference READING:    From Ingot to Target: A Cast Bullet Guide for Handgunners by Glen E. Fryxell and Robert L. Applegate with a Forward by John Taffin