I work in Yakima with a local Historical Council to identify and maintain local historical land marks and markers. Because brass thieves have stolen many of the old brass markers, and because there is little interest in maintaining these sites, they are typically destroyed or in poor repair. While looking for materials to replace older text plaques we considered Lexan, wood, and other sign materials. As an interim sign, to be used while we identified a suitable material and procured funds for it, I volunteered to try and make a low fired bisque clay tablet sign with hand lettering.
My idea was to work with a clay slab 18” X 24” and about ¾” thick. The technical challenges of making this sign are well known to tile makers, who can easily make a green clay tile but have difficulty in drying it so it remains flat. The lettering of this type of sign is relatively easy as long as you work on the damp clay slab surface before it dries.
I first made about a half dozen sample signs over a twelve month period. I found I could dry 12” X 12” slabs suitably, but larger slabs required constructing a “Tile press/drier frame” which I call a “sandwich drying frame” (or sandwich).
The drier is constructed from two pieces of ¾” plywood, cut to the same size. I then made a sandwich using these two plywood plates on the out sides (the bread) and layers of drying material between (to which the moisture away from the two sides of the slab equally to keep the slab flat), in this order: ¾” plywood, thin sheet (or two) of packing batt (the cushion) the same size, piece of plastic sheet, piece of burlap with an inch of overlap on all sides, and (next to the clay) a piece of cotton canvas the size of the frame. Next comes the clay slab. Over the clay slab, the sandwich layers are laid in the reverse of the other side. You can handle these slabs by grabbing the whole sandwich, with clay slab inserted but for larger pieces, the weight of the wet clay, plus the sandwich is relatively heavy. To solve this problem I have two belts which I wrap around the whole thing, both ways, and I can then move or transport the loaded sandwich, flip it, or work with it.
Once the sandwich dryer pieces are cut, lay the bottom plywood, foam, plastic, burlap, and canvas on a table, roll the clay slab and Lay it on top of the canvas. Then finish the sandwich. Grab it and see if you can turn it over. If so, you are ready to make the sign. By the way, this is also a good way to make smaller signs and large clay tiles. A 12” X 12” press would be simple to handle, but a 36” X 36” is approaching the weight where it is hard to handle (and requires a larger slab roller). The larger the piece the thicker the slab must be and the longer it will take to dry. The challenge of creating larger signs increases exponentially with size.
I roll my slabs out on a North Star slab roller which only goes out to an extreme of 24” X 22”. In my opinion, because the completed slabs will be exposed to some stress in a public setting, I make them about ¾” thick. Thicker slabs increase in weight quickly but thinner slabs become too fragile. A bone dry slab must be handled very carefully but it is possible to work with it on the foam layers. Never move the slab without the sandwich support boards and padding.
It can take up to 6 weeks to dry a large slab so it remains flat (with this method), depending on size, as follows:
1. Don’t waste the back of your sign. Look at both sides of your wet clay slab and choose the front, or work both sides and choose the finished one you like best. I choose the best clay surface for my finished piece and perform shrinkage measurements and draft wording and draft lettering and designs on the back. No point in wasting a perfectly good surface.
2. I usually let the clay dry in the sandwich frame a day or two before starting lettering. Clay slabs seem to “set up” in the first day or two of storage and it doesn’t hurt to start this slab drying slowly.
3. Your sandwich should allow the clay to dry the same on both sides to discourage warping, but I “encourage” the drying by laying the sandwich flat on a table, and removing the layers down to the clay on one side, letting it dry an hour, then reassemble the sandwich turn it over, and let the other side dry an hour, then reassemble the sandwich and let the moisture content stabilize for 24 hours. The sandwich allows the slab to dry uniformly (read “flat”) over time, but the attention you give it can speed up this process. Understand that all handling exposes the slab to irreparable accidents.
4. I often put a bit of weight on the sandwich during drying times. The slab can remain in the sandwich for long periods of time without attention. Though technically it is drying outward from the center, complements of the canvas wicking the clay moisture outward, the slab should dry the same on both sides and thus remain flat. The secret of drying a large slab perfectly flat is (in the absence of complicated machinery) taking your time. Consider at least a month (or more) necessary for drying a large, flat slab in this manner. If the dried slab has a bit of wobble, don’t stress it in any way as it may break. A few mm off perfectly flat is suitable if the slab is cared for and then glued to a plywood back when finished.
5. Specifically designed and constructed drying frames with fans and turners will probably speed up drying, but I like this method because it is easy, cheap, and effective. However, it is not fast. If I’m busy I can leave the frame unattended for a week and not worry that it is creating warps. I have several frames and work at drying several slabs at once. Work ahead of your projects.
6. I have successfully stored dried slabs on their sides when strapped in the sandwich. If you try to store them without support, expect some loss. You can fire a bunch of signs in one kiln, horizontally but protect them while they are waiting to be fired.
7. If this drying sandwich has an engineering flaw it is that it could make a slab dry with a crown, as it will, during the drying progress, be wetter in the middle than on the sides. The two burlap wicks on both sides should stabilize this crown and you should, if you take time and care, end up with a very flat tile. However, these dry slabs do have some wobble on a flat surface, so get it glued to a piece of plywood as soon after it comes out of the bisque kiln as possible.
8. Consider fiber filler to add strength to the bone dry ceramic plate. The fiber may be pressed into the back of the sign to allow the sign surface to retain it’s workable nature on the front. The fiber will burn out in bisque and is primarlly for strength in the green clay phase.
9. I haven’t experimented with multiple slabs drying in one sandwich but there is no reason it won’t work if canvas wicks are laid evenly between clay slabs. However, this would add weight and make individual slabs harder to work with.
Bisque firing and sign mounting
When the slab is leather hard, it is nearing the pint where it is stable and will not buckle with uneven drying. I take the slab all the way to bone dry before I remove it from the sandwich. I continue to dry both sides to open air for an hour or so a day (depending on my schedule, no big deal either way) until the bone dry stage is reached.
The next step is finishing the dry slab. You can work on it with sand paper and clean it up some, but it is very fragile now and any handling requires the utmost care. I transfer it to a shelf and then carry the shelf to the prepared kiln.
When you are ready to move the sign to the kiln, lay it on a flat plywood batt and carry it to the kiln. Prepare the shelf so there are no shelf overlaps under it in the kiln. If there are, use stilts to support the shelf directly below the slab. Don’t build slabs which, when dry, are larger than your kiln.
Once the sign is bisque fired I epoxy it to a piece of plywood for instillation strength. I also seal the edges of the installed bisque tile with silicone and treat the surface with Thompson water seal or other ceramic/concrete weather treatment. If water is absorbed into the bisque sign surface and frozen (not far fetched occurrence for outside monuments) the frozen water in the sign will break out and leave a surface picket in your sign. THIS IS NOT GOOD. Try to install it where rain, snow, and wind do not reach it, such as under a hood or eve. I also suggest periodic weather treatments to outside signs. Clear glazes will also address this problem, but experiment with test signs to avoid destroying weeks of labor.
One mounting that I was satisfied with was to epoxy a sign to a finish piece of plywood several inches larger then the sign, sealing the edges, drilling holes in the protruding plywood, and then screwing the plywood to a bulletin board. You can also glue a bisque sign to a prepared stone or concrete surface.
Except for the practical detail of better lettering, I am satisfied with this type of sign as a permanent placement. Avoid trying to mount the sign with screws through sign face holes. Expansion and contraction will soon overstress the holes and your sign will break. When epoxied to plywood and sealed it should survive vandalism, weather, and time. For a historical marker, it has a burnished leather look, and can be very effective for historical themes.
Here are other ideas for “tile signs” uses
- Markers on historic homes and buildings (12” X 12”)
- Trail markers, using oxides to enhance lettering
- Land boundary markers with text
- Memorials
- Landscape directions for workers, for example: an irrigation pipe map, directions for servicing drainage lines, etc.
- An effective award can be made from a large bisque plate.
- Garden signs.
I wish to thank J. N. Miles for his kind suggestions and valuable historical perspective with this project.
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