by Brian Kerkvliet copyright 1995
In the last few articles I focused mainly on the development of style and various goings-on in the lampworking world. Now I think it’s time to get back to some of the nuts and bolts of flameworking! In my experience as a teacher and someone who is plugged into the “bead zone”, I have noticed a growing trend of bead makers shifting their focus more towards other ways of working glass over the torch. This is very exciting, because I believe there is a lot more one can do on the torch besides beads. Don’t get me wrong; beads have their place in the glass art world. I enjoy doing them because they give me a way to experiment with a lot of different techniques, processes composition color and design, on a small scale. These can then be translated to other glass canvasses. Working with beads also gives you some kind of connection to the history of glass. One of the natural outgrowths of bead making, both historically and now, is the formation of core vessels. I hope to show you how to take your bead skills and apply them to making core vessels.
Core vessel overview
Creating a core vessel on the torch is very much like making a very large bead, only the hole doesn’t go all the way through the piece. Core vessels are essentially glass formed around a central clay core material which has been shaped on a mandrel of some kind, either a large bead mandrel or a blow pipe depending on the size of the finished piece. The formed clay is then cased with glass either by dipping into molten glass, or by coiling the glass around it. Another way is to coat the core with wet frit and melt it onto the surface, kind of like Pate de verre in reverse. After the piece has been shaped, decorated and annealed, the core is removed. Wa-la, you have a vessel!
Building up the core
There are several ways you can create a core form for your vessel. The ancient core vessels were believed to have been made with a concoction of clay and cow, or perhaps camel dung. In this case the clay would have provided a binder and the straw in the dung would provide some air space within the core matrix so that the core could be removed easily. It also adds to the aroma of the process. If you wanted to avoid using this aromatic mixture, I suggest trying this mixture:
20% ball clay
20% alumna hydrate
35% silica sand 100 Mesh
enough finely cut dry straw to make up the balance
You want to mix this up with water, so it resembles a bread dough-like consistency. This mixture is then formed around a stainless steel mandrel wire that is bent into a mellow zig-zag shape at the end (ill.-1). This helps to keep the clay from breaking free from the mandrel, after you get the glass wrapped on it.
Another way I have made my core is with fine grade steel wool. This method is attributed to the late Kisao Iburi who shared it with me when he taught up at Pilchuck a few years back. Essentially you unroll the steel wool ball so you have a long batting. Take this batting and lay it out flat on a table. Then take your mandrel, place it at one end, and start to roll up the steel wool. It is important that the wool is wrapped tightly onto the mandrel. To aid in getting it tight, you can use a small piece of 2 X 4 or a brick. Take the brick and place it on top of the mandrel which has the steel wool partially wrapped around it. Press the brick down on the mandrel, and gradually roll it forward, winding up more of the steel wool. This trick helps to compress the wool tightly around the mandrel so it doesn’t break free later on. As you roll it on, you may notice some wool may poke out loosely on the end of the rod. Fold over the loose bit so it gets reintegrated into the shape of the form. You may have to do this several times over the course of the winding. Once the bulk of the wool is wrapped onto the mandrel, roll the shape between your palms so as to assure an even, centered shape.(ill.-2) Be sure to smooth in any loose bits of steel wool by rubbing it into the final form. You want to make the overall form slightly larger than the portion that will be coated with glass. Once you get the form centered, and the way you want it, then coat it with your favorite kiln wash or bead release and allow to dry. When the coating reaches a plastic-like state during the drying stage, wet your finger and smooth out any bumps or irregularities on the form. Then allow it to dry the rest of the way. This smoothing process is a good thing to do for both the clay and the steel wool methods. It is theoretically possible to sculpt the core or put a design texture in the surface before letting it dry completely. This is something that I have often thought about but haven’t played with too much. If anyone tries it, let me know. Both the clay/ straw and the steel wool cores should be soft enough to allow the glass to contract slightly during cooling, and yet give a strong enough form to shape on. If the core doesn’t have enough give, the glass could crack on cooling.
Now that you have your core made, it is a good idea to burn it out in a kiln. This fires on the clay and burns out the straw, and-or oil that is on the steel wool. Place the cores in a kiln with their handles extending out of the door so you can grab them after they have burned off. Vent the kiln so the fumes have a way to escape, and take the kiln slowly up to about 1000°F, and soak it there for about 15 minutes.
GET A HANDLE ON IT
I like a handle on my mandrels because the piece tends to get large and heavy. A larger diameter handle makes it easier to work with. There are several ways you can put a handle on. The easiest is to use a piece of dowel and drill a hole in it so as to accept the mandrel wire. The drawback of this method is that the handles can’t go into the kiln! They also tend to burn out after prolonged contact with the mandrel, which tends to get hot after a while. However, they are cheap and readily accessible. Another option is to use a pin-vise that has the correct cullet size. These can be found at jewelry suppliers. Often they have four different cullet sizes, a reversible one on each end of the pin-vice. These can be loosened up and slipped on and off of the mandrel quite easily, so you can put it on when you take the core out of the kiln, and take it off when you put it into the kiln. All of the pin-vises I’ve seen are too short for my hands so I chopped one in half and welded an extender stainless steel tube so the whole handle is about 6.5 inches long.(ill.-3)
THE GLASS WRAP
Now it’s time to apply the glass! Take the core out of the kiln and start to heat it in the torch. Use a large bushy flame at first and gradually dial in a hotter flame. If the core material starts to pop or crack, slow down and don’t heat it so fast. The torch is much hotter than the kiln so you want to take your time heating it up. You may also notice some gases or fumes coming from the core as well. Just keep heating until all of the fumes are gone, and the core looks a fired white-gray color. When I ‘m making core vessels, I put the ends of several sticks of the primary background glass into the kiln for preheating. This allows me to quickly melt the glass onto the core as I go. When I use up the preheated portion of the rod, I place it back into the kiln and pick up another preheated rod. This makes the first body wrap relatively quick compared to the amount of time you would have to take if you were heating up the rods from room temperature.
The glass is applied in much the same way as you would make a bead. Starting at the top of the core vessel, (that is closest to the handle), apply the glass to the core material and slowly wind it around the core so as to overlap on to the previous layer. If you get any bubbling coming up through the glass, there is a good chance that you didn’t sufficiently burn off the core. If this happens, stop applying glass and continue to burn off the core. Work out any bubbles before you start applying the glass again. You want to build up at least 1/4 inch of glass evenly around the core so that it has some strength to it. I try and make the glass thicker at the bottom of the piece for stability and balance. You want to be careful that the top of the vessel has a nice edge on it, so it is not sharp! I don’t know about you, but I don’t like to do any more grinding than I have to. The best way to make a nice even lip on your vessel is to do a wrap slightly back from where you want the lip to be. Then add a few more wraps of glass on top of that wrap. This section of glass is then heated and marvered so the edge of the lip roles over and makes a nice smooth face. (ill.-4) After you get the basic form, you can decorate it with any number of methods. Many of them are used in bead making, such as feathering, dots on dots, cane drawing, shard application, trailing, murrine application, adding preformed
sculptural components that were waiting in the hot annealer to be applied. The list is endless! The thing you need to be aware of is keeping the whole piece hot while you are working a smaller part of it. This means you need to heat up the whole piece evenly first, then go in and do your detail work for a short time before resuming to heat the whole piece. By going back and forth in this manner, you can achieve a lot of detail without thermal stressing the piece. Generally it is a good idea to work on the portions of the piece that are less prone to thermal shock before going on to more sensitive items like feet,
handles or attached parts. A foot can be made by coiling up glass on the bottom of the piece, much the same way that a coiled clay pot is built. (ill.-5) You do need to be sure that all of the seams have been worked out from between the coils, otherwise the foot has a chance of cracking. A graphite paddle marver with a slight radius to it is very helpful for performing this task. After the whole piece has been shaped and the decoration applied, bring the whole piece up to a uniform temperature before removing the handle and placing it into the annealer. If possible, it is a good idea to stand the piece upright in the oven so there is less chance for it to deform. If you can’t place it upright, make sure the piece is cool enough so it won’t distort when it’s placed into the oven on its side.
REMOVING THE CORE
After the piece has cooled to room temperature, take it out of the kiln and inspect it for cracks or blemishes. If you notice anything strange, it may be possible to fix it by leaving it on the core and slowly bringing it up to temperature in the kiln. Then take it out of the kiln and quickly get it into the flame to heal the crack or blemish. (More info can be found on this in my column “Magic Kiln Tactics” Glass Art, March /April ’95).
Hopefully there is no need to perform this task, and you can proceed in removing the core material. Gently wiggle the mandrel until the clay starts to break away. Continue until you can get the mandrel out of the piece. Then with a small dental tool or other scraper, try to remove as much material as possible. After you have cleared the bulk of the material out of the core, mask off the entire piece with heavy masking tape. Then cut a hole in the top so you can sandblast away any remaining core material. Remove the masking and admire your new core vessel!
BLOWING THE LID!
Now that the body of the piece is finished, you may want to make a lid for your urn. This will keep the magic potion in, or the evil spirits out. Or it may simply make the piece look completed. The creation of the lid can sometimes be as challenging as making the piece itself. You first need to see if the opening of the piece is round or not. If it looks oval or otherwise out of round, you may want to wait for one that is round before attempting to make a lid. In other words, you want it to be as round as possible to match the lid up with it. I prefer to blow the lid using a borosilicate tube for the blow pipe generally about 10 mm medium wall. By rotating the tube in the flame, it will thicken slightly. Use your flaring tool to flair open the end just a little. (ill-6) Let this cool down enough so that you can easily coil the soft glass around the end without melting the borosilicate glass blow pipe. Don’t worry about joining these two incompatible glasses because when they are hot they will stick to one another, and when you want to remove the blowpipe from the lid, it is easy because the soft glass will melt at a much lower temperature, so the borosilicate will pull right out, like a stick out of a mud puddle.
Wind the glass on evenly so the wall is nice and thick throughout the bubble. You can change colors midway if you do it evenly, by tapering off the first color and tapering on the second color. (ill.-7) After you have the glass wrapped on in a little light bulb shape, marver it lightly and even up the walls. Work out the seams between the coils by alternately puffing and marvering the piece. (ill.-8)
After you have evened out the wall thickness, make a neckline with the edge of your paddle and start forming the shape you want the top to be. You want the collar on the neck to be the same size as the opening on the vessel, (ill.-9 dimension “A”) . Measure it with you calipers so that it is slightly larger than the opening. This will later be ground to fit to insure a tight seal.
Blow out the shoulders of the lid so they are roughly the same dimensions as the outside of the vessel. (ill.-10) – (ill. 9, dimension “B”) If you need to build up the outside edge a bit, you can wrap another layer of color around that portion or add a twisted decoration cane. Shape this ridge so it now measures exactly the same as dimension “B” (ill.-11).
After you get the dimensions where you want them, heat up the tip of a soft glass rod to use as a punty. Lightly attach the punty to the top of the lid and heat the joint where the borosilicate and soft glass meet (ill.-12). When they get hot, gradually pull them apart until the soft glass severs from the borosilicate blow pipe (ill.-13). After you take the blowpipe off, heat the opening and flair it out with a small graphite reamer (ill.-14). Open it up enough to match the size of the opening “A”. Check and recheck it with the calipers! Now you need to transfer the punty to the other side of the top so you can firepolish the top of the lid. Heat the tip of the glass rod and attach it to the rim of the lid insert. After you stick it on, gently pull the rod a little thinner and jog it over slightly so it is on center with the punty on the other side. Now remove the first punty and firepolish the top (ill.-15). Heat the whole piece evenly one last time before knocking it off into the hot annealer. To knock it off take your tweezers and chill the joint where the punty joins the rim, then lightly tap the punty with the tweezers. It should fall right off into the annealer!
When the piece has cooled down, check it for fit with the vessel portion. It may be necessary to use some 100 grit silicon carbide mixed in a slurry to grind the lid so that it makes a good seal. Place a little of the slurry on the rim of the vessel and place the lid in place. Gently rotate the lid back and forth on the vessel until the lid grinds into position on the vessel. It may be necessary to apply more grit from time to time as you grind it down.
With these basic techniques you can explore many different possibilities available to you at the torch. I hope that these tidbits will be of interest to you. Let me know what you think and send me slides of your pieces!
For more on core vessels, check out these books:
GLASS 5,000 YEARS By: Hugh Tait,
AN ILLUSTRATED DICTIONARY OF GLASS By : Harold Newman