February 17, 2008

Finished palstaves and some additional notes on casting

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I carved a fair amount of the first sand mold using hardwood pottery tools. They worked well, but wore down pretty quickly. I switched to a set of dental tools, which don’t wear down as easily, but don’t necessarily speed up the process. I could see making a set of bronze tools for carving the material. One of the tools has a small spoon shaped end that is perfect for cutting even and consistent flanges.
The depth of various parts of the mold is measured with a bamboo skewer. I shared a studio with a potter who used them to make sure that his pots were uniformly sized and borrowed the idea. I can make a mark on one skewer to measure the depth and then move it over to the duplicate half of the mold and place it on the mirror image of the first place. A second skewer is laid across the mold, perpendicular to the upright measuring skewer. This easily shows me how much deeper I have to carve.
The sides of the molds are scored with a hacksaw to make sure that they are properly aligned for casting.

After carving a couple molds I’ve found I can turn out a set of them in about an hour. Although they aren't permanent molds, that’s not bad considering that these molds hold up much better than the clay molds I made last semester.

February 10, 2008

Two Palstaves

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The two palstaves after a little bit of cleanup.

Freshly cast axes

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Unfortunately this type of mold tends to crack as the metal cools. However, they are still a great substitute for sandstone.

Resin Sand Molds Ready for Casting

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Casting Axes - Part 3

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Delta resin with silica sand mold for a palstave.

This semester I began with a new mold material. Delta resin with silica sand and catalyst. This is a resin based medium that duplicates soft sandstone. It’s easily carved, even with wooden modeling tools and has a texture that is remarkably like sandstone. Unfortunately the resin breaks down at high temperatures and while it makes a great mold, it still is only good for one pour. On the plus side the carving goes quickly and I don’t have to wait for it to dry as I did with the clay molds. It didn’t take long to make molds for two axes and they were ready to cast with only a little preparation. To make a smoother surface I mixed graphite and zircon (50/50) and added enough denatured alcohol to make a thin solution. This was painted inside the mold and then lit to burn off the alcohol.

The molds did not require warming and after clamping they were ready to cast. This time I used a standard bronze of 90% Cu and 10% Sn. The pour went beautifully and the axes need very little finishing. I was able to break off most of the flashing with my fingers and used a grinder to take off the sprue. I had expected some shrinkage and so had made the molds slightly larger than the models. However, there was no shrinkage so I’ll have to allow for that for the next pouring. I now have a set of axes as soon as I can find some serviceable branches to make into hafts.

Notes: In order to estimate the amount of metal needed, sand is poured into the mold and then weighed. The weight of the sand x 5.3 = the amount of metal needed for casting.

References: Potratz, Wayne 2005 Hot Metal (Minneapolis: Turtle Sign Company)


January 26, 2008

Checking the pit furnace

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The second furnace is dug into the ground and lined with firebrick. The hose from the vacuum cleaner goes in at the open end at the lower left of the photo. The steel tray next to Wayne's knee is filled with sand and is used to support the mold while the metal is poured into it.

Molds and branches to use for hafts

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Clay molds for a palstave

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Two halves of a clay mold. The white material is talc, which is used to make it easier to pull the mold apart. Separating the mold to get the model out is one of the more frustrating parts of the process. If the mold sticks, or the model sticks inside the mold, the clay has to be repaired. Vaseline is also an option.

Firebrick furnace

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Casting Axes - Part 2

When making the molds, we tried different clay bodies, different tempers, and tried them with and without sifted manure. The frustration level with making these sorts of molds is high. If the clay is too wet, the molds crack. If they dry too fast they warp. And the recipe I use one month might not work the next month. When I began the semester in September the weather was still warm and humid. By the end of the semester it was near 0º F and so the heating system was on and the air was very dry.
I did manage to get a couple good axes cast. The department has a set of Chinese hand bellows that are used for demonstrations, but because there is limited time for class, we reverse the polarity on shop vacuums and use them as blowers. Two double chambered furnaces were built of firebricks. Charcoal was heaped around the crucible and the mold was kept in the second chamber in order to keep it warm. Having the charcoal heaped over the crucible meant that the charcoal worked as a flux for the bronze. When the crucible was pulled out covered with foreign material one person had to skim the slag off of the metal while the other poured.
Afterward we shattered the molds with hammers and scrubbed off the metal. I did use a grinder to take off the excess metal rather than take the time to use mud saw or a line with grit as it might have been done in antiquity.
In the 2008spring semester I plan to make some permanent molds in order to cast multiple axes.


October 18, 2007

Casting Axes - Part 1

In August of 2007 I contacted Wayne Potratz of the Unversity's art metals department. He teaches a primitive casting course there and was interested in working with me on making working duplicates of some of the axes in the collection. I began by making plasticine models of the originals.
In September I began sitting in on the primitive casting class. The first order of business was to process the clay we would be using to make molds. Dry raw clay was pounded and then sifted into finer and finer grades. One student who had more experience sorted the nodules of clay. He grabbed handfuls of inspected them and then tossed some of them away and had us pound and sift the rest. Since I was a newcomer, I had an easier job of sifting grog, ground fired clay that is added to the regular clay to prevent shrinkage. There was dried manure that also needed sifting, but it was decided that the student who didn't show up would have that job. I made a mental note to show up as often as possible.
The sifting was done with mesh screens, but in earlier days it was probably done using baskets or by flotation. One of the graduate students realized that more clay needed to be processed than we could possibly have finished and so threw a sheet of plywood over a pile of the dried clay and then ran a forklift over it a few times. It led me to wonder how someone in the Bronze Age would have done the same job using an ox or two. We had a group of twelve students working steadily. In addition to the processing we also wet down clay and stirred it to make slip. This seemed like a good sized group to get enough material processed to provide for molds for everyone until midterm.

October 6, 2007

Poster

In April 2007 I created a poster of the university axe collection.

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Flanged axes

Two of the University's flanged axes.

Axe Data including measurements

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This file contains data about the individual axes including weight and measurements. The data on the metals has been limited to those above 1%.
This is raw data taken with a NITON XRF analyzer. Please be aware that the data includes surface corrosion and may not reflect exact alloy analysis. I will be posting a more indepth analysis. I hope in the near future I will have data from microprobe analysis that can be compared to the data given here.

Axe Data - Metal analysis

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This is the full Excell worksheet of the axe data. The first column sorts the data by the axes' accession numbers. Readings were taken from both the haft end and the blade. Some of the axes have third readings taken from points of interest.

This is raw data taken with a NITON XRF analyzer. Please be aware that the data includes surface corrosion and may not reflect exact alloy analysis. I will be posting a more indepth analysis. I hope in the near future I will have data from microprobe analysis that can be compared to the data given here.