Scratch testing applications with Boart Longyear

by Jon Peterson, Product Manager – Diamond Products at Boart Longyear

Selecting the right diamond coring bit for the ground you are drilling can be challenging – especially when navigating the product selection from different manufacturers. From crown height to material hardness, coring bits represent a refined and complex technology designed to produce precision exploration data.

By leaning heavily on the industry standard hardness scale — MOHS — there are tools that can save drillers time and money. For more than 200 years, MOHS has been proven science that drillers and geologists may rely on and utilize to reduce guesswork in the field.

The secret? Every driller should start with a scratch test kit. Understanding the MOHS hardness scale and the use of a scratch test kit will result in more accurate bit recommendations with every test. The scratch test helps take the speculation out of selecting the best bit.

Since its development by the German mineralogist Friedrich Mohs in 1812, the MOHS Hardness Scale has been used to characterize a mineral’s resistance to being scratched or abraded by another mineral. The scale of common minerals is shown in the chart on the right. For example: a crystal of fluorite (MOHS value of 4) will scratch a crystal of calcite (MOHS value of 3) but not a crystal of quartz (MOHS value of 7).

Diamond is the benchmark for the hardest mineral on the MOHS scale due to the rarity of the naturally occurring and manmade minerals that would scratch or abrade diamond (see lonsdaleite or wurtzite boron nitride). One of the most abundant minerals in the Earth’s crust is quartz (and its variations), which has a MOHS value of 7.

Scratch test kits are comprised of four pencil-shaped tools with pre-sharpened tips on each side representing hardness values from 2 to 9 and a sharpening stone (to reshape the points from wear). These alloyed tips offer ease of identifying the associated MOHS hardness value.

Get started

Once you have the kit in-hand, grab an early core sample or adjacent rock and follow these steps to greatly enhance your success in choosing the right bit:

1. Clean the surface of the rock to be tested.
2. Hold the core sample and firmly press point to scratch onto rock.

3. Brush away any fragments, powder, or debris that is produced.

4. Check to see if a distinct groove was made in the rock (not just a mark that can be wiped away).

4.a If a groove is questionable take the 2-value scratcher and lightly drag across the scratch to see if it catches.

5. If no scratch is made, then move up in number to the next scratcher in the set.

6. Once a scratch is made, conduct a second test to confirm results.

Scenario one

You encounter a competent hard rock formation while on a surface drilling project utilizing an LF160 Surface rig and need to collect more information to pick the correct bit to continue drilling operations. After retrieving the core sample, you take out your scratch kit and begin to test with a Number 5 scratcher, which produces no mark on the core sample, and proceed until the Number 7 scratcher makes a possible mark and the Number 8 scratcher makes a definite mark. The rocks MOHS value is between 7 and 8. Initially, you would select a Yellow Longyear bit, but there are other factors to consider before choosing the correct diamond bit. You should also look at the competency of the rock, the grain size, and quartz content to determine if the Yellow Longyear® formula is a correct choice.

This rock is competent with a high amount of medium to coarse size grains of quartz. With the high amount of quartz grains a tougher formula should be selected to decrease wear on the matrix supporting the diamonds. A Green Longyear® bit would be the correct selection to balance life and penetration rate. The competency of the rock formation, in this instance, will aid in selecting the correct waterway design.

Longer life? Pick a Stage® 3 design.

Faster penetration? Pick a Tapered Swirl (TSW) design.

Scenario two

Before starting an underground project, you are allowed to visit the core shack to examine core samples from previous exploration projects to understand the geology of the area you will be drilling. At periodic intervals you take scratch samples of the rock to determine the most common MOHS hardness and what is the highest MOHS scale mineral configuration you will encounter. The results determine most of the rocks are granite-type formations. The goal for this project is focused on penetration speed. Selecting a Red Longyear® formula with a TSW waterway design will maximize speed and improve the life of bit.

These examples are for ideal situations; however, ground conditions may vary by region and even by drill sites in the same location. Boart Longyear offers a scratch test kit to help identify the hardness of the formations being drilled. For assistance in selecting the right bit for your drill site, contact your local Boart Longyear sales and technical teams.

For more information visit: www.boartlongyear.com