archaeometallurgy

These are the main modes of material removal from a sample.

This is part five of the series on slag microscopy and deals with sample lapping. In the lapping process samples are also ground, but with fine abrasive powders. The abrasive grains can roll between the sample surface and the lapping disc. The section receives a dull finish. Lapping is especially effective in maintaining the flatness of the samples and and edge definition of samples consisting of composite materials with hard and soft components .

For this we need:

• lapping disc or a 20 by 30 cm piece of 8 mm flat glass
• abrasive SiC powder, e.g. FEPA F800 (ca 6.5 microns)

Sample lapping?

I found that one lapping step between grinding and polishing resulted in better polished sections. It allows allows you to stop grinding after paper grit 1200. This step is very simple to do manually and does not take long:

Squirt a smallish amount of water on your glass lapping plate. Put a small amount, e.g. half a teaspoon,  of F800 SiC powder near the water puddle. Now use the sample to draw in some SiC powder into the water and move the sample block in a figure eight motion across the glass plate, drawing in more abrasives as you go along. During lapping the abrasive grains will break down to smaller particles and produce a finer and finer surface. You can see this effect when compare the sample surface that was lapped with fresh abrasive with one that was lapped with old abrasive that was used for a longer period of time. The surface lapped with the old abrasive will be finer.

Lapping does remove less material than grinding. You can,, however, determine the material removal rate by the ratio water to abrasive powder, i.e. the viscosity of the lubricant film. The thicker the film, the less material is removed. If your film gets too thick, the sample will only slip across the plate. If it too thin the sample will get stuck on your plate due to strong adhesive forces.

You will develop a feeling for this process rather quickly. You can also hear it, if material is removed or not…

Recap

• Lapping is a simple, optional step
• it may shorten prep time, because you do not need to use the finest grinding steps
• maintains flatness and edge definition
• removes linear scratches resulting from grinding

References

Highly magnified schematic view of a cross section through a sample during the grinding process. Grinding serves two purposes, first to remove micro-shattering caused by cutting (1 and 2), second to prepare the surface for polishing (3 and 4). With consecutive smaller grits you have to remove the scratches left by the previous grit.

Hello to part four of the slag microscopy series. After mounting, it is about time to start sample grinding to prepare the section for polishing.

For this we need:

• a sample polisher/grinder
• abrasive discs
• optional: stereo-microscope to inspect grinding progress
• optional: a marking gauge

Polished section needs to flat. Bottom and top surfaces of the section have to be plane-parallel. A good polish is only achievable if the surface has undergone adequate preparation. This is done by grinding on a horizontal water lubricated  abrasive disc. Flatness is usually achieved with abrasives bonded on paper, in synthetic materials or in metal. Continue reading

This is part three of the series on slag microscopy. Today is about sample mounting: we use a cold mounting procedure, i.e. samples are mounted in resin. I have talked about  the find documentation process  and the cutting of the sample in parts one and two.

Sample mounting: Three different sizes of sample cups: 20, 32 and 40 mm.

The following things are needed for this:

• cold mounting resin and curing agent
• mixing cup
• sample cups
• ultrasonic bath
• IMS
• optional: exsiccator and vacuum pump
• safety goggles
• lab coat or an apron Continue reading