A Guide to Air Abrasives

a-guide-to-air-abrasives-graphic.jpgAir abrasive fossil preparation powder

Air abrasives can transform your prep both in terms of quality and speed. An air abrasive unit (it’s essentially a bench-top sandblaster) is used to direct a stream of compressed air containing an abrasive powder (often called the abrasive media) through a handpiece at a fossil-bearing matrix.

Some air abrasive units are purpose built for fossil preparation (keep your eyes peeled – we have one coming out for a very specific job soon!), and others are commandeered from the dental industry or industrial applications such as cleaning electrical parts. Micro blasters, air abrasive units, miniature sandblasters, micro-abrasive blasting, pencil blasting or whatever you many call them have found their way into the hands of hobbyists for not only fossil preparation, not only fossil preparation, but for etching and carving materials as diverse as plastic, metal and glass.  They are used in archaeology for cleaning metalwork and in industry for cleaning and polishing precision components.

Air abrasives are usually used for finishing or fiddly work and are most effective when the fossil is harder than the rock that encases it. The results when the right powder at the right air pressure is used can be astounding. Air abrasives can allow for a finish that wouldn’t be possible with an air pen alone or allow you to do a job where vibrations of air penning may damage the fossil. In some circumstances, air abrasion is the only technique that can do the jobThe following series of photos give examples of prep jobs that have been made possible by air abrasives; either there was no other way of preparing them or a superior finish has been acheived. 

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All professional workshops contain an air abrasive unit (or many with each containing a different powder for a different purpose); and increasingly home set-ups are utilising air abrasives as good quality units are available at more affordable prices (such as the Vaniman Mobile Problast). 

Here's a quick visual breakdown of an air abrasive set up. The air consumption on air abrasives can be moderate to very high depending on the model, and so a more powerful compressor with a larger tank than would be used for air pens alone is required. Between the compressor and the air abrasive unit air filters are required to remove moisture from the airlines. All work should be done in a blast cabinet with suitable dust extraction. 

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Air abrasives are used by holding the handpiece like a pencil, and using a foot pedal to control when the powder is flowing (a bit like a sewing machine). Powder flow and force can be controlled by adjusting the air pressure at the pressure regulator (mounted somewhere on the unit). Low pressures will naturally provide a more gentle flow and allow more delicate work. What is considered low pressure will vary depending on the machine. 

The angle at which the jet of abrasive powder is held is very important. A 45° angle can act to gouge, which is great when digging through matrix, but less good when working to the fossil. This can also rebound within the gouge and end up eroding the opposite side too. Low angles (25° or lower) are usually best when working on fossils. Not only do you avoid the risk of directly gouging, but you can also work to dislodge matrix from underneath. The spray pattern can also be modified by holding the nozzle closer to or further away from the fossil. A less powerful, more dispersed spray pattern can be acheived by holding the nozzle further away, and a more concentrated, accurate flow can be maintaned by keeping the nozzle close. Always keep the jet of abrasive moving rather than holding over a single part of the fossil for too long. As with all fossil preparation tools, it is very easy to damage the fossil. All abrasive powders run the risk of obliterating detail. Care must be taken, usually to prevent contact between the fossil and the powder. 

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Some air abrasives are fitted with pinch valves between the foot pedal and the powder tank. These allow more control than those without. For instance the Vaniman Master Mobile Problast is fitted with a pinch valve, or instant off mechanism which does just that – the airflow stops immediately as the air hose is pinched to stop further flow. When a machine is not fitted with a pinch valve you sacrifice accuracy of work, as the flow of powder gradually bleeds off over a few seconds. The hosing of machines with pinch valves will need replacing more often, as the repeated action of clamping down on it will wear it through.

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There are a variety of different powders out there to put in an air abrasive unit. The most common in fossil preparation are dolomite, aluminium oxide, iron powder and sodium bicarbonate. These vary in effect as a result of the differences in grain size, grain shape and hardnessMany professional preparators will say that it is worth getting to know a powder and knowing it really well. You can then achieve just about anything with said powder by understanding which pressure and angle to use it at. All powders are capable of obliterating detail on a fossil, but it is worth trying to match the powder to what you are preparing. Some powders are more forgiving than others on different surfaces, and so experimentation and experience are the key to success.

The ideal scenario is a powder that is harder than the matrix bearing the fossil, but softer than the fossil itself. This, in theory, will lend itself to removing rock without damaging the fossil. Practically this is seldom the case. To give some kind of guidance on powder hardness (or agressiveness) they are measured in terms of Moh's Hardness. This is a relative index used in Geology and Mineralogy, using a scale of 1-10 with one being the softest. 

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  • Dolomite is a very popular powder in fossil preparation. It strikes a balance between matrix removal and gentleness on the surface of the fossil, with a Moh’s Hardness of 3.5-4. It is particularly effective on shale, siltstone and limestone. The grains are predominantly angular, providing a gentle cutting actionWhilst it is enormously effective in fossil preparation when it’s flowing well, dolomite powder is often associated with clogging, clumping and producing large amounts of dust. It also generates a great deal of static and therefore clings to itself, the internal plumbing of your abrasive unit, and the sides of the tank. All that aside, many preparators find dolomite to be totally worth it! Recycling is inadvisable as when the particles collide in the airstream, they break each other down to very fine particles. Works well on limestones and sandstones.
  • Aluminium Oxide (or aluminum for our American readers!) is used as a cutting agent due to the angular grains and it’s score of 9 on the Moh’s hardness index. Typically used for speedy roughing work and matrix removal and smoothing. Not recommended for use with delicate or fragile fossils as it can obliterate detail very quickly or destroy the fossil altogether. It can be used exceptionally well but (as with other powders) runs the risk of giving the surface of the fossil a tell-tale ‘melty’ or frosted look – a bit like glass etching on crystalline surfaces. However, at very low pressures with great care it can complete more delicate tasks. Flows really well even with some humidity. Can be recycled 2-4 times. Effective on all rock types.
  • Sodium bicarbonate is the gentlest option for fossil prep. It suits very soft rocks and fragile fossils well. The grains have a monoclinic shape (needle like particles) which allow it to cut soft surfaces like shale and chalk, and some softer limestones. It can also be washed off a fragile fossil easily as it dissolves in water. Moh's Hardness 2.5-3. Not recommended for recycling.
  • Iron powder has a very different character to the white powders. It works particularly well on moderate to soft fine-grained matrices such as limestones. The high density spherical grains ‘pummel’ or ‘peen’ the surface using a percussive action to hammer away the rock. It acts, when used carefully to preserve and slightly polish crystalline fossils (such as the calcitic shell of an ammonite); and is often able to cut through the rock without damaging the calcite or removing ammonite shell. It can darken the surface of a fossil (which may or may not be the desired efffect!). It is 4 on the Moh’s Hardness scale, but can appear to be more aggressive as a result of the density. Not to be used in an abrasive unit that uses an electromagnet (magnet being the operative word!). Fines can be explosive and may be ignited by heat, friction, sparks or flames and so care must be taken. Highly recyclable. The fines will be extracted away (but do not help much for fossil prep) and the larger particles can be swept up from the base of your blast cabinet. Do bear in mind that iron powder by nature can go rusty when in contact with oxygen, therefore clumping together and potentially causing a clog.

Not all abrasive powders are created equal and care should be taken in where you purchase it from. Typically, these powders are not used for fossil preparation or even microblasting and so will not necessarily have high levels of quality control - if the price is too good to be true, it probably is! The grain size is a really important factor, as is the particle size distribution (the difference in size from the smallest to the largest grain). Dolomite in particular, can have a very wide range of particle sizes (usually too many fines) which further prevents it from flowing, and if there are too many fines can prevent it from doing its job.

All abrasive powders should be sieved before use, to remove any larger particles. Having a good quality sieve will serve you well. Many of the cheaper sieve meshes are between 50-100 microns out, which either means you will end up throwing away good powder if the mesh is too fine, or introducing too large particles to your air abrasive system.

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All air abrasive units should be fed with exceptionally clean, dry air to prevent powder clogging (or rusting and clumping in the case of iron powder). When air compressors draw in air, the air is compressed until it reaches a set pressure above normal atmospheric pressure. When this happens, moisture or humidity in the air condenses out. This usually gathers in the bottom of the air compressor tank (which is why you drain it), but some of it will end up going into your airlines and towards your air abrasive unit. Whilst it usually wouldn't do the unit itself any harm, moist air will render owning a unit fairly pointless as you won't be able to get a good, consistent flow. This is particularly true of powders like dolomite and iron powder, which are extremely moisture sensitive. Aluminium oxide is probably the most forgiving powder.

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The Fossiler's Dropout Air Filter Kit, complete with Dropout filter, Carbon Filter and Pressure Regulator.  Filter kit installed in a workshop, used between the compressor and the tools to make sure that air is exceptionally clean and dry. Thank you to N & S Hollingworth for the photo.

 

This can be achieved by installing a Dropout® air filter (and a carbon filter if working with dolomite or iron powder) which removes 99.9999% of liquid water from the airline going into your abrasive unit. Before the advent of Dropout® filters, many people would use a stack or series of air filters or water traps, followed by a coalescing filter to remove oil vapour (pretty much all air compressors powerful enough to run air abrasives are oil lubricated). A complete series of good quality air filters decreasing in size from 40μm, 10μm, 5μm, 1μm through to 0.01μm plus a coalescing filter will set you back hundreds.

We worked with Dropout® to develop an air filter kit perfectly suited to the demands of the fossil preparator. This kit serves as an all-in-one solution to replace these air filters that represents better value for money, easier installation and much easier and less costly maintenance. The complete kit is recommended for use with moisture sensitive powders like dolomite and iron, and a single dropout filter is more than sufficient for bicarb or aluminium oxide.

You might come across dessicant and refrigerant driers online, but don't worry - these are very much overkill for fossil preparation!

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It is important to match the grain size of the powder you’re using (measured in microns) with your air abrasive tank and nozzle. The orifice size in the tank (the outlet hole for the powder) needs to be the same or very similar to that in the nozzle. If you have a smaller nozzle size than the orifice then there is too much air going through the line than the nozzle will allow, resulting in a clog. One thing to note is that the larger the orifice and nozzle, the higher the air consumption – this is why you might hear about iron powder (typically 150 microns) is quite greedy on the air and requires a larger compressor! The larger the nozzle that is used, the more air and the more powder comes out, and the risk of losing some of the velocity as the particles knock against each other.

Nozzles are typically lined with embedded crystal or tungsten carbide. Crystal embedded nozzles have a slightly wider spray pattern and so therefore aren’t as accurate as tungsten carbide. However, they are twice as durable and will last longer. Tungsten carbide allows for greater accuracy but will wear faster.

The Vaniman Mobile Problasts come with a series of interchangeable tanks and nozzles, purchasable separately in 25-50 micron (dolomite – very well sieved for even flow), 50-100 micron (aluminium oxide, dolomite), and 100-250 micron (iron powder) sizes to allow you to use a variety of powders on the same unit.