Author Topic: First Delft clay casting--account and tutorial  (Read 10195 times)


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First Delft clay casting--account and tutorial
« on: September 23, 2017, 08:10:03 PM »
Greetings all,

After several months of planning and procrastination, I just completed my first backyard brass casting using Delft clay. Results were better than I could have imagined, even on the first try! Here's an account of the process.

I am working on a circa 1700 English horse pistol, based on pistols similar to this one:

I particularly liked the sideplate and thumbpiece on this gun and wanted to put something similar on my piece. Obviously, you can't just order these things from TOW or MBS, so I either had to cut them out of sheet brass or mold them myself. I decided to do the latter. After doing a lot of internet research, and with some advice on several points from Dave (Smart Dog) Person, I collected materials for Delft clay casting. Most materials came from Rio Grand jewelry supply or Amazon.

The first step in the process is to create a full sized model of the piece to be made. I started with a sheet of hard green jeweler's wax to create a model, but found this material to be very difficult to carve and shape, fragile, and difficult to repair when breaks occurred. I gave up on the wax and used a different material I've never used before--high density polyurethane foam board. Jim Kibler recently posted about how he uses this material to make his masters, calling it "butterboard". I got a 12" square x 1" thick piece of this material from this site:

The butterboard has a weight and feel similar to balsa wood, but has no grain and no tendency to split or crack. It can be sawed, carved, and shaped using chisels, gouges, files, powered burrs, or any tool you can use to shape wood. It is very slightly flexible and quite strong and resistant to breakage. It doesn't melt with heat or chemical solvents. I found it amazingly easy to carve and shape with considerable detail.

I started by creating full sized drawings of my planned parts on paper, then cutting out the drawings and gluing them onto a 1/8" thick slab of the board, cut on a band saw. The edges of the paper pattern were "stabbed in" with curved gouges, and the excess cut away. Piercings were cut out roughly with a burr on a Dremel tool and finished with needle files. After the outer contour was established, the paper pattern was peeled off and details sculpted in using small gouges, V-tools, and files.

The last picture above shows my mostly-completed butterboard models next to the disastrously bungled hard wax model I first started with.

The casting itself started with preparation of the Delft clay. I got a nice heavy loaf of Delft clay from Rio Grande. Placing the clay on a cookie sheet, pieces were cut off and chopped into kibble with a wide scraper or trowel. The kibble was then used to fill the lower half of a cast iron two-part mold I bought from Amazon. As the clay is added into the mold, it is compacted firmly by pounding with a hammer. Here, the picture shows me using a rawhide mallet. I later switched to a medium-heavy ball-peen hammer. A small squared block of wood was used with the hammer to compact the corners and edges. After completely filling and compacting the clay into the lower half of the mold, the top surface is smoothly sliced off with the wide scraper or trowel.

The master model is then placed firmly face down into the lower half of the mold and very firmly pushed halfway into the clay. Check the impression and make sure the detail is well transferred into the clay. The master is then replaced into the impression and the surface of the mold dusted uniformly with mold parting powder (basically talc). Blow off any excess talc, and check the impression in the clay. I removed stray bits of talc from the impression and the mold surface using a very soft artists brush. After replacing the model into its recess, the upper half of the mold is positioned and is filled with Delft clay, using the same technique as the lower half.

The two halves of the mold are then separated and the model removed. A pouring hole and air vents are then made into the upper half of the mold. I passed a drinking straw from the center portion of the mold impression all the way though to the top of the clay to create a pouring sprue. Air vents were then created by cutting little channels about 1/16" wide in the clay, extending laterally from the outer projecting details of the impression to allow good brass flow into these details. Small holes are then made from the outer ends of these air channels up to the top surface of the mold using a finishing nail or 1/16" drill bit. The top mold is then turned over and a cone is cut away around the pouring sprue hole to allow an easy pour. All the air vents are checked to make sure they are clear of debris. Any loose clay that could fall into the mold during pouring is carefully removed. Also check the mold impression cavity to make sure no debris has entered it. The two haves of the mold are then put together. Ready to pour!

I had first anticipated melting my brass in a biggish graphite crucible in my electric Neycraft furnace, but after reading other people's accounts, I decided to melt the brass in a small porcelain dish crucible using oxy-acetylene. Others have used MAPP gas, but acetylene is hotter and quicker. I made a simple open melting oven outside using some regular red bricks, creating a little chamber surrounded on five sides by the bricks to help hold in heat during melting. My casting brass came from Rio Grande as small shiny square brass nuggets in a cloth bag. You could use scrap brass, but I wasn't sure about the composition of the scraps I had on hand, so went with the pure stuff from Rio. I had previously glazed my porcelain dish crucible with borax (see recent separate posts). I placed the spring holding wand around the crucible and placed it into the oven recess, then used my small portable oxy-acetylene torch with a rosebud tip to heat and melt the brass.

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It goes without saying that you should use all safety precautions before doing this. Here, I'm wearing leather welding gloves, a leather apron, a long-sleeved wool shirt, and a face shield, and also have a big bucket of water and a fire extinguisher nearby.

After the brass appeared molten, a little borax based flux was sprinkled into it and stirred with a graphite rod. A small amount of slag floated to the top and was removed with a perforated spoon. The flame was kept on the molten brass as it was quickly tranferred to the mold and poured quickly and smoothly into the sprue hole until the cone was filled. A little bit spilled over the top.

After the flames died down, and waiting several minutes to cool and solidify, the mold was opened. Success! A completely filled mold! The Delft clay was broken out around the brass casting. I threw away the small amount of burned and blackened bits of clay, but saved and recycled the rest for later use.

The casting was then taken back inside to the bench where the excess brass in the air vents was cut away with nippers and the pouring sprue was cut off with a hacksaw. Small bits of flash at the outer edges were cut off with a belt sander. Bits of flash in the piercings will need to be cut out with needle files later. The surface of the casting is slightly rough compared to the master model and will need cleanup with riffler files and small stones. The excess pieces of cut-off brass will be recycled for use in the next pour.

The thumbpiece will also have to be gently curved to fit the contour of the pistol butt before inletting.

Flushed with success after this first pour, I decided to proceed with the sideplate. The sideplate master model was too long to fit in the cast iron mold, so I used a homemade mold I had made previously with 3/4 x 2" pine wood. The two halves of this mold are firmly aligned with modified removable large nails passed through drilled holes. For the sideplate, I packed the wooden mold with Delft clay and followed almost the same procedure as described above, except that I made two separate sprue holes instead of one because I was concerned that the long length of the piece would lead to premature cooling of the brass and an incomplete fill if poured only through one central sprue hole. Consequently, I used a straw to create two separate pour holes, one in the front-mid and one in the back-mid portion of the mold impression, angling these holes to meet close together at the top of the mold. A single pouring cone was then cut out, joining both sprue holes to allow both to fill from a single pour. As before, multiple air vents were created around the outer edges, especially targeting small projections and thin portions.

Melting and pour were then carried out in the same fashion. Here's the result after opening the mold. Another complete fill!

Sometimes, shrinkage of cast parts can occur, so I left a little excess metal around the screw holes of the sideplate in case this happened. As it turned out, the size of the casting was almost identical to the model and appears to fit the stock sideplate panel perfectly! The final picture shows the two cast pieces next to their master models. These masters could be used again to make more of the same castings. Again, some flash within the piercings needs to be cleaned out, and the surface cleaned up with rifflers and stones. I'll post pictures of the pistol when done.

Here is a link to a you-tube video showing the Delft clay casting--pretty similar to what I did except that I used acetylene.

Hope you enjoyed this!


« Last Edit: January 13, 2020, 05:35:26 AM by rich pierce »