Firing/finishing terra-cotta/ceramic

[fritchie]

I didn't think I had seen anything like the title subject in this forum, but I felt compelled to search anyway. The closest I came to what I need is a remark in a report on Elizabeth CAtlett that she has used terra-cotta, among other materials.

I’ve just committed to firing 4 small pieces in what basically is earthware or a sort of terra-cotta for an upcoming Miniature Show at my old art school, an annual invitation affair. I realized a while back that the 4 small bronze torso’s I had planned would not be ready in time, and made the decision to transfer to a low-fire ceramic to shorten production time. As the ideas basically were ready, the actual sculpting went very quickly, and the pieces are to be delivered for firing on Tuesday (2 days from now).

My questions basically are about finish. The school instructor, who will do the firing, says he generally paints his pieces with acrylic - a base color and then a brushing with very different colors to bring out textural variations, if I understand his approach. My inclination is to leave the material natural, if it fires a fairly uniform color, but possibly to coat it with clear shellac or vanish to give a cleanable surface. Is this possible with “earthware”? (I’ll use that term, as I don’t really know what appearance to expect.) The clay contains some fine grog, is a deep red in color when wet, and is said to fire at cone 06 - 04 to a mocha color - medium to dark brown, I presume.

Although it’s already too late to worry about the method of clay handling, I’m also concerned about the small bits I added in many places to alter shapes. The 4 pieces each consist of a flat panel, about 3/8 inch thick and 4 ½ inch square, with a high relief male or female torso as center of interest. The deepest part of the figures is about 2 inches, and they are partly hollowed from the back.

Any comments on finishing (with paint, varnish, or shellac), and the proper way of adding or subtracting small bits of clay in making sculptures of this sort is welcome. I did do the obvious thing, I think, of wetting the area each time I added clay, and the pieces currently are air-drying very slowly.

[jwebb]

If you've adhered the added bits as you describe, and gotten them well-dried, and the firing sequence is right, there should be no problem. The main risk I've experienced with terra cotta is cracking, usually caused by bringing the temp. up too fast. I think most kilns today are controlled by plc's (programmable logic computers), so it's even less risky than when I used to do a lot of this. While working as a studio assistant 20 years ago, I once fused a whole kiln-load of students' work together into one huge biomorphic blob. Nearly burned the building down, too. The surface will be like that of a "bisque-fired" pot, before the glaze is put on, if you've seen that. It's quite versatile. I've seen pieces painted with washes and water colors and various matte finishes, and also with metallic paints, to simulate bronze. I personally like the salmon color of the clay itself, with maybe just a thin white wash on it for highlights. I think you're going to love it.

[fritchie]

Thanks, jwebb, for these comments. I'm expecting a matte, open-pored surface. This school doesn’t do glazes; its mainly an early learning option. When you say washes, what material are you describing, thinned acrylic or something else? This clay probably will be a medium to dark brown, so probably only fairly thick applications of color, or dark colors, will be effective. Maybe also a thin application of white, as you say.

I’m a little concerned about “closing the pores” with a water-impermeable material like shellac or varnish, but I like the idea that it might ward off grime. Do you think this would be a problem? And, I’m sort of also assuming that a heavy coat of acrylic would be water-impermeable when dry. Do you know if this is so?

On adhering new clay, that’s pretty much what I did. The pieces all now are a light terra-cotta, similar to 4 test squares I made the first day and have completely dried. The instructor says he “cures” the pieces for about 12 hours below water’s boiling or flash temperature of about 212 F, before heating at 100 F/hr., so firing should be fairly gentle. If only they don’t melt! This is not the regular school clay. He says he occasionally has gotten some pieces partly to melt. Tomorrow’s commitment day, so I have my fingers crossed.

[jwebb]

We always fired at cone 06, which is what you say you're using, and melting was never a problem (except for the one time I messed up the loading and blocked the shut-off mechanism from working correctly.) As I recall, cone 06 is about 1925 F, which shouldn't melt anything. I've only used water color washes or a thin "slip" of clay, but have seen a wide variety of finishes on fired pieces. Unfortunately I don't know exactly what was done to achieve them. I think, though, it will accept about any finish you want to give it. It's not exactly "porous". I'll be curious to hear about your results.

[fritchie]

According to a seemingly authoritative reference I found on the web, cone 06, the lower in the description I was given, is about 1830 F, and cone 04, the upper, is about 1940. Each step seems to be about 50 degrees. The instructor says he usually fires at 1950, just about the upper level recommended for this clay. That’s why I am a bit concerned, but I don’t know the properties of this sort of ceramic at all.

I watched as he loaded the kiln today, probably to start the low-temperature, 12-hour drying an hour or two ago. He had 3 clay figures from the current class, each about 15 - 18 inches high in a seated pose, and he had room to put my 4 small panels and 4 test squares on the floor of the kiln. At least that should be the coolest place, if there’s any significant variation.

I told him about your comments re washes and watercolor, and he said that he tends to avoid watercolor because any variation in the composition or permeability of the fired piece will magnify itself in a wash or transparent color like watercolor. He also said that, in his experience, these pigments cannot be lowered in intensity (or removed partially), but only deepened. He seemed to feel that shellac or vanish wouldn’t be a problem at all, if I choose to use it.

I plan to keep options open until I see the final pieces, but if the color is uniform, I may just leave the pieces as is, or maybe change the luster a bit with a clear coat such as vanish, shellac, or “silk/gloss medium”, which I think essentially is a polymer, maybe polyurethane, varnish. Thanks for all the comments. I should be able to get the pieces Friday pm, and I’ll report the results.

[fritchie]

jwebb - I got the 4 small pieces from the 1950 F firing today, and they are fine, but much lighter than I expected. They are approximately a common adobe color. Right now, I think I’ll stain them a darker color, with a bit of irregularity in the shading. Hopefully I can post a picture.

[Aurora]

Wax

Melt parafin (careful, it may catch fire) or I use a dark brown industrial wax. Melt the wax in a turkey pan on the stove. Ladle the wax (or dip) and if the wax cools too fast and leaves globs, put in a standard over, about 250F until the excess runs off. This seals the pores, makes it easy to clean, and sometimes darkens the piece uniformly.

When I use acrylic to finish, I first shellac the piece so the acrylic goes on uniformly and I have more control.

[fritchie]

Thanks, Aurora, for this suggestion. The pieces had to be turned in yesterday (Monday) and I finished them in acrylic. I used a mix of yellow ochre, cadmium red and cadmium orange for the base color - a couple of runs to get variation and complete coverage. Then I spotted the pieces with pure Hooker’s Green. This went on much heavier that I intended, but I decided to let it stay as is, because of the overall feel of energy. I didn’t have time to take photographs, but should be able to get an installation shot.

What do people recommend over the acrylic, if anything? I bought a “matte medium” polymer, intending to use it for protection, but on reading the label, I saw that it was not recommended as a finish coat. I had clear wood varnish, but decided not to use that. I know people apply a thin varnish coat over oil for protection, but what is done with acrylic? Especially on ceramic such as this?

[Aurora]

Brush on Floor wax if you want a bit of a shine, or butchers wax if you want a matt finish. You can apply butchers wax to your hands after you coat the entire peice and rub it on the surface so that only the highlights have a bit of a shine

(I sound like a wax salesman)

I like the waxes because it seals the pours, makes it nicer to touch, you clean the piece by running water over it and it has a more human/earthy look to it (not industrial looking)

[fritchie]

What is this butchers’ wax, and where do I get it? I bought slabs of a yellowish, fairly hard wax used to coat wax paper for food processing quite a few years ago, looking for a substitute when I had trouble getting regular casting wax. Might that be what you are describing? It's probably too hard to rub directly, but it probably could be softened with a little naphtha (carefully, flammable!!!)

[Aurora]

The butchers wax that was given to me years ago is in a can and a paste (pale yellow). I imagine what you are talking about sounds the same but in block form. Try thinning it, its worth a try.

[Aurora]

(Here I'm back talking wax again):p

I use bees wax for my wood carvings to give a matt, yet shiney finish where I polish. Same effect as butchers wax but expensive. You could use a cheaper melted wax to seal the pores and a bees wax for highlights (I'm going to try that next time)

Bees wax in paste form is readily available at any place that specializes in fine finishes for wood.

[fritchie]

Thanks for these replies. While we're on waxes, maybe you can help me with this: My art school had a guest lecture from a patinist with a large east coast foundry about 11 - 12 year ago, about the time I was taking my last classes, and I was very impressed with the wax he used in finishing example pieces. He said it was an artificial beeswax, mixed with naphtha to make a paste. I found some artificial beeswax and made a similar paste for myself, which I used until it ran out 2 - 3 years ago. This wax went on the warm bronze very easily, and polished to a high, hard sheen.

Since I ran out, I’ve tried many different waxes without finding anything I consider as good. Only one art supply store locally said they had seen artificial beeswax in a catalog, but eventually they said they couldn’t find any. I tried mixing a little real beeswax with candlewax or hobby wax, but the result had a gummy feel, and the beeswax (in the jar of the mix) over time turned it almost a fluorescent yellow - quite bright, and not to my liking. The earlier mix remained completely clear.

Any ideas on good waxes? This is for bronze, and I prefer a clear, hard, high-polish coat.

[kaidu]

I know that this is a bit late,but...

There are many commercially available ceramic glazes that can be used for low-fire clay like terra cotta. Another way to finish a clay piece is to burnish the surface with a smooth object (i.e. the back of a spoon or a smooth pebble) It is kind of time consuming,but will yield a smooth and somewhat shiny surface without any further work. Painting ceramics is ok as long as you are sure that it will remain dry.

[fritchie]

Thanks for these suggestions. This may have been a one-time affair, but I enjoyed it and possibly may do more.

The school instructor who fired the pieces asked when he saw the raw, dry clays whether I had burnished them, because the surfaces were fairly smooth and shiny. That was just from working the wet clay, however, because I did these about the same way I normally would for casting in bronze. I wound up using acrylic, and am happy with the result.