I treated seven pieces of iron from a larger collection excavated at Ham Hill hillfort in Somerset, an Iron Age site. The archaeologists wanted the iron to be cleaned for interpretation rather than display, so we were able to practice on them without too much pressure. To clean them, most of us used an air abrasive machine.
Air abrasion is like sandblasting. We use aluminium oxide, which is a finer powder than sand. It is dangerous to breathe the powder, which gets everywhere, so masks must be worn. By the end, your clothes are covered with a fine layer of powder. The objects to be treated are placed boxes with a viewing window and hand holes, which is hooked up to an air extractor. The aluminium oxide is blown through a small nozzle, which is held at a shallow angle to the metal surface. The airflow is pedal-powered, and the air pressure and amount of powder can be controlled at the machine.
The iron objects had large, bulky corrosion products. The rusty orange or brown layers were removed by air abrasion. The fine aluminium oxide powder knocks off corrosion, turning it into dust and blowing it away. Its hard to see the powder working, so it looks like the corrosion is melting off, worn down by the powder.
There is a dark grey layer that follows the original surface, called magnetite. The corrosion products have to be removed, stopping at the magnetite layer but not hit the metal core underneath. This is fairly hard in practice, as its hard to see in the boxes and with the poor lighting. The stylus is held at a shallow angle so that higher areas are removed. This can produce a relatively smooth surface if done right, but my surfaces were fairly rough. Every once in awhile, you have to pull out the object to see if you are at the right level or not. There are also often voids and blisters in the iron. These areas can pop off if you are not careful, revealing the metal beneath.
One of my objects was a scythe. The scythe had dusty orange corrosion and large blisters. I slowly wore away at the corrosion with the air abrasive machine, and removed a couple of blisters by snapping them off with dental pliers. The corrosion abraded off fairly easily. It took about 4 hours to abrade the entire piece. Although it looked like a dolphin at the beginning, the shape and edges of the blade were visible after treatment. The surface still had voids and blisters, which I had avoided, but the object could be identified and its shape studied without much problem. The end of the piece had fallen off at some point, and I adhered it back on with epoxy.
.bmp)
Another object I treated was a broken horseshoe. I knew it had holes from the X-ray, and I knew that three of the four had dense metal inside. Therefore, when I abraded the piece, I was able to look out for them. The corrosion was harder than on the scythe, and so it took longer to treat the horseshoe. In the end, it took over 5 hours to treat. The surface was rougher and more uneven than the scythe. However, the shape and characteristics were evident.
.bmp)
I cannot say that I enjoyed this treatment. I found the air abrasive machines difficult to use well. It was slow, hard to see, and dirty. The set up caused back pain because I had to bend over the boxes for hours. I was quite glad to be done. However, it was a good skill to learn. I certainly am not great at air abrading yet, but with practice, I will improve.
Air abrasion is like sandblasting. We use aluminium oxide, which is a finer powder than sand. It is dangerous to breathe the powder, which gets everywhere, so masks must be worn. By the end, your clothes are covered with a fine layer of powder. The objects to be treated are placed boxes with a viewing window and hand holes, which is hooked up to an air extractor. The aluminium oxide is blown through a small nozzle, which is held at a shallow angle to the metal surface. The airflow is pedal-powered, and the air pressure and amount of powder can be controlled at the machine.
Top: the air abrasive machine (top left), box (bottom middle), and extractor tube (right). Bottom: horseshoe being abraded.
There is a dark grey layer that follows the original surface, called magnetite. The corrosion products have to be removed, stopping at the magnetite layer but not hit the metal core underneath. This is fairly hard in practice, as its hard to see in the boxes and with the poor lighting. The stylus is held at a shallow angle so that higher areas are removed. This can produce a relatively smooth surface if done right, but my surfaces were fairly rough. Every once in awhile, you have to pull out the object to see if you are at the right level or not. There are also often voids and blisters in the iron. These areas can pop off if you are not careful, revealing the metal beneath.
One of my objects was a scythe. The scythe had dusty orange corrosion and large blisters. I slowly wore away at the corrosion with the air abrasive machine, and removed a couple of blisters by snapping them off with dental pliers. The corrosion abraded off fairly easily. It took about 4 hours to abrade the entire piece. Although it looked like a dolphin at the beginning, the shape and edges of the blade were visible after treatment. The surface still had voids and blisters, which I had avoided, but the object could be identified and its shape studied without much problem. The end of the piece had fallen off at some point, and I adhered it back on with epoxy.
Top: Scythe before treatment. Center: X-ray of the scythe, showing dark voids, a thinner edge, and an irregular surface. Bottom: after treatment.
Another object I treated was a broken horseshoe. I knew it had holes from the X-ray, and I knew that three of the four had dense metal inside. Therefore, when I abraded the piece, I was able to look out for them. The corrosion was harder than on the scythe, and so it took longer to treat the horseshoe. In the end, it took over 5 hours to treat. The surface was rougher and more uneven than the scythe. However, the shape and characteristics were evident.
Top: horseshoe before treatment. Upper middle: X-ray, showing the holes and metal in the first three holes. Lower three images: after treatment.
I cannot say that I enjoyed this treatment. I found the air abrasive machines difficult to use well. It was slow, hard to see, and dirty. The set up caused back pain because I had to bend over the boxes for hours. I was quite glad to be done. However, it was a good skill to learn. I certainly am not great at air abrading yet, but with practice, I will improve.
