|Monthly Tech-Tip |
Pyrometric cones were first developed in 1896, they are still indispensable in ceramics, there is nothing like seeing a correctly bent cone to verify that the kiln fired to the right temperature. Cones have always been readily available but now, due to supply chain issues, we cannot be guaranteed of being able to get them. In some countries the expense of cones has become a cost burden. Amazingly these can be made at home! This project will be ongoing as we learn and develop the recipes and process to make the cones.
It was easy to design the 3D geometry and print press molds. We have been working on ways to press the clay into the molds and determine the best plasticity and stiffness. Casting is another option, this is looking even more promising. Using a 3D printer it is easy to make PLA molds to produce the needed plaster molds.
A challenge will be the bending range. An Orton cone 6 bends through a ~30F degree range. This may be a product of tradition or it may be technically desirable. Or it may be better for a cone to complete its bending in fewer degrees than Orton has done since we don’t use guide and guard cones anymore.
Print these three, pour plaster into them (after soaping) and you have a cone slip casting mold. Part 2 has a separate upper to enable printing it upright without support structure (producing a much higher quality surface). Hold that top cap on with a rubber band to cast. The separate cap also makes it easier to extract the plaster mold after set. If you would like this 3D file in Fusion 360 format, it is available in the Files manager in your Insight-live.com account.
Supply chain problems are hitting everything. Even cones. It turns out one can make these! The shape of a self-supporting cone is easy to draw in 3D (I did it here in Fusion 360). It is a 25mm equilateral triangle base lofted to a 3mm one 65mm straight up on the front side. And then a cut-out across the front. By using 3D printed press molds and plastic clay I can press these by the dozen. What about a recipe? Cones melt short of being glazes but beyond being porcelains. I chose L3685Z3 engobe as a starting point, it has a linear vitrification curve spanning a wide range. Approaching this on the material level, not as a chemistry project, I did three iterations of adding Ferro frit 3110 to the engobe. Shown here are the second, "A" and third, "B" (on the right is an Orton cone 6). B has too much frit, A does not have enough. You likely guessed what I did next: Mixed A and B. The result was almost perfect, bent just a little too much. If you would like this 3D file in Fusion 360 format, it is available in the Files manager in your Insight-live.com account.
This is recipe L4532D. We are still working on the molding process, whether to press the plastic clay or pour slip clay into a plaster mold. Or both. CAD files are available in the Files section of your Insight-live.com account so you can 3D print your own cone molds or shell molds. Pressing into the 3D printed PLA forms is potentially much faster and easier if it can be made to work. The issue is that the pointed ends are quite delicate and either crack in the mold or break during handling. The cross-section is also a very uneven thickness and requires special techniques to prevent cracking or warping during drying. The casting process is working better, the cones are more durable and drying is not an issue. Mold release has been a problem but we are finding that using ball clay instead of kaolin produces a better casting and releasing slurry (for example, the L4532F recipe).
The rear cones are Orton 5 and 6. The front ones are the L4532F recipe, it is bending too much at six and not quite enough at cone 5. The L4532F recipe employs ball clay instead of kaolin, which is making for better casting properties and better dry strength. It has also greatly reduced the cost, removing the need for Veegum. The difference in bending for this one-cone range is also looking similar to what an Orton cone would do.
Orton says “90 angular degrees is considered the endpoint of cone bending”. First, let's assume the normal: Examination of cones on kiln-opening to verify controller operation. Consider the cone on the left: The tip is touching. But it is also beginning to buckle, which means it was touching for a while before the firing ended. Who knows how long! The second one is not touching but has still fallen a little too far. Why do we say that? The third one, positioned on the Orton guide, has reached the recommended 90 degrees. This demonstrates a good reason why self-supporting cones are much better than standard ones: They are not touching when considered done. And standard cones, when sent in a 3/4" plaque, have a less consistent bending behaviour.
People refer to the extent of cone-fall as numbers-on-the-clock or degrees. This cone is at 5 oclock or 80 degrees. Notice that from start-to-finish is 35 degrees C (not all cones have this same 35 degree fall). As you can read on the temperature scale, 25+ degrees happen before it reaches 2 o'clock! From 5 to 6 o'clock is only 1 degree! This is a standard cone that requires a plaque, notice that the down-touching position is when it hits the top of the plaque. It follows from this that one can convert cone-bend to equivalent temperature. That being said, remember that cones measure heat-work, so the conversion is only valid for a 60C/hr rate-of-rise.
|By Tony Hansen|
Follow me on