Cone 04 Terra Cotta Casting Body, Transparent Glaze, Firing Schedule

Project Name

Casting strong and durable terra cotta at cone 04

Project Codenumber

UnAssigned

Notes

Terra Cotta is being discovered by many potters (and hobbyists with interest in production) as being stronger and more durable than they thought. And firing at cone 04 is so much easier and faster than for stoneware. And the colors available are much brighter than at high temperatures. It seems there are thousands of commercial glaze products for use at cone 06-04. And it is just as easy to mix your own glazes.

Plainsman Clays makes a body named L215, we are attempting to make a casting body that fires to similar colors, of similar maturity and working with the same glazes.

But there are two big obstacles for those interesting in production: The expense of the jars of glaze and the time-consuming process of painting them on. What if you could mix buckets of your own base clear and white glazes? And dip bisque ware to get even coverage and quick drying? And at a fraction of the cost. Then use commercial paint-on products to decorate the ware? Then later, if you ramp up your production, you could buy stains and make your own decorating products. But is it possible to mix your own ultra-clear base that could equal the quality of the ones you buy? Yes.

That is part of what this project is about. But there is more, we are not just talking about the glaze. What if you could cast terra cotta ware quickly and efficiently. Light and strong. And all the glazes worked perfectly on it. What if you could make your own simple plaster molds using a 3D printer? Can you see where we are going with this? In an exciting direction. Stay tuned.

L215 Terra Cotta Casting #1

Total:144.96

Notes

*Note: This recipe is superseded by L4170B, a 90:10 blend of Redart/Ball Clay to get faster casting. However the notes below are still applicable.

I have tried many terra cotta casting recipes in the past but decided to start over with the simplest recipe I could think of: 100% RedArt! RedArt is a fabulous red-burning terra cotta widely available in North America, it is redder than any other clay I know and has been consistent for decades. And it matures at a very low temperature. As it turns out, it is not plastic enough to have the strength to pull away from the mold. So I added 20% ball clay. The result is something amazing! I can cast 3 mm thick in 10 minutes and have the piece out of the mold in another 10. A dry in front of the fan in another 20. The KT1-4 ball clay is not slowing down casting, yet adds so much to the leather hard handling strength and dry strength. If you do not have a large particle ball clay and have to use OM4, for example, then consider using a 90:10 ratio instead (otherwise casting might be too slow).

So far, this has proven to work well with my G1916Q clear glaze. I have used a simple firing schedule that ramps up to 1830 quickly, then slows down to 108F/hr to 1920, holds for 10 minutes, then shuts off (you would need to use a cone in your kiln to discover at what temperature it reaches cone 04). I will graduate this to a drop-and-hold firing schedule if it becomes necessary. I will also be testing more at cone 03. For thinner ware there is a tendency for some shapes to warp, so that is a signal that the body is firing to good strength there. But I am suspicious that cone 04 will be best production temperature for me.

The fired strength of this body at cone 04 is excellent. It is not a stoneware, but it far stronger than a white-firing body of the same temperature range. I think there would be very few utilitarian tasks this combination would not be up to the challenge for. And the warm red color is outstanding. The potential for volume production of ware with this body, the G3879 glaze and cone 04 firing is tremendous.

This recipe very easy to mix, I just added the Darvan to the water, poured in all the powder and it mixed in 20 seconds. Read the paragraph below, this recipe is a suggestion. "Listen to your slip" rather than this page if it is not deflocculating right.

2000g of powder produces about 1.8 liters. It is best to mix the slurry to a workable consistency first (not too thin). Then work with it until it gels and remix, adding a little more Darvan. You may need to do this on several occasions before it stabilizes. I used Darvan 811, it is supposed to be better for high iron slurries (to prevent gelling), but I am not convinced that it actually works better than regular Darvan.

Pictures

G3879 Clear on L4170 TerraCotta Casting

The clear glaze is G3879. The white on the outside of the one on the left has 10% added zircopax. The overglaze colors are Spectrum Majolica colors.

L4170 TerraCotta with G3879 zircon white

The insides of both are done with a 10% zircopax-added version of G3879. I compared it with Spectrum Majolica white (on the outside of the one on the right, it had to be painted on), the potential to get a much more even coverage is there because we can dip-glaze this one. The outside color tests on the left are Crysanthos Underglazes.

L4170 at cone 06, 02, 1 (bottom to top)

Cone 1 is stoneware strength. Cone 02 is the warmest color. But cone 04 (not shown) is also pretty good.

G3879 at cone 1 on a terra cotta body

This combination qualifies as a stoneware, the body has less that 2% porosity at this temperature.

Fired Redart, Tuckers, M2 bars cone 02, 04

Top to bottom: L4170 Redart casting body L4183 Tuckers terra cotta L4184 M2 throwing body

L215 Terra Cotta Casting #3

Total:144.96

Notes

*This reduces the amount of ball clay in L4170 to speed up casting time (at risk of mold release and tearing). This has proven to work well. The casting behaviour is still good and release is quicker.

We are testing a variety of glazes and underglazes on it with good success.

This is producing an amazing stoneware starting around cone 1. And getting better toward cone 4! More info and pictures coming.

At a 1.79 S.G. this recipe will produce about 4 litres of usable slip from a 5000 gram batch of powder.

Comments

2021-03-02

Joe: Jan. 4/21
After 10 minutes mixing time.
S.G. was 1.78
Ford Cup was 58 seconds.
Next day: Ford Cup increased to 1:15 and 1:20. After 10 minutes in mold, slip surface began to cloud over at about 8 minute mark, but not total distortion before pouring out. Bowl released within 2 hours and had a nice smooth surface in and out. Cutting lip of bowl displayed minimal tearing of surface, this is good.

2021-03-02

Joe: Repeat of above mix Feb.8/21
S.G. 1.79
Ford Cup 1:02 after 15 minutes mixing time. Checked bucket 2 days later, and no water sitting on surface, mixed for abut 20 minutes and ford cup reading was 2:02, so has more or less doubled.
Casting wonderful as usual, a 25 minute cast seems to be quite nice.
Cast 2 mugs for 25 minutes and added handles made from L215 P6877 run about 2 -3 hours later and used slip to join handles to mugs. This slip adheres like super glue. Mugs dried slowly over the weekend with plastic/rags and there is no indication of cracking at any point where handle joins mug. This casting body also seems to be quite durable with high green strength?

2021-03-02

Joe: Feb. 22/21 doubled recipe to 10 KG. powder quantity, mixed briefly and let sit overnight. Next morning no water sitting on surface of slip in 20 litre pail, so, even more surface area than approx. 7 litre Canadian Tire Bucket. This amount mixed quite well with lab mixer, but any more and the large mixer would be required, and was used for initial mixing first day.
After mixing approx. 15 minutes S.G. 1.79 and Ford Cup 2:01 so cast two terra cotta molds and slip is beginning to show light distortion on surface at 12 minute mark. Cast pieces 25 minutes and ware was out of molds within 75 minutes.
After casting about 2 litres of slip over the course of a week at a rate of 25 minutes in planter molds, checked S.G. once again and is constant 1.79 but ford cup would barely not empty at 2 minute mark so added 3 grams darvan to remaining 6 litres of slip and ford cup reading is now 2:07, but slip is slightly warm so this might be aiding in flow. Cast once again at these readings.

Pictures

G1916Q on L4170 cast pieces

G1916Q on L4170

Thin application is clearly the way to get the best transparent.

Cone 04 L3685Z2 white engobe with clear glazes

The colour as amazingly white. And this engobe appears to be fitting very well even though we are dipping pieces, like a glaze, to bisque ware.

Handles are L215 P6877 pugged clay

Handles are L215 P6877 pugged clay attached to mugs with the L4170B slip which acts like glue, a minimal amount required to attach handles. No cracking evident anywhere around handles.

L4170B at cone 04, 2

Very dense and strong at cone 2, it’s fired surface is almost velvety, very pleasant to touch.

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L4170B bars fired cone 04-5 (bottom to top)

White Engobe #2

Total:124.50

Notes

*This is the product of a long development program that culminated in the realization that a super-white engobe requires using super-white kaolins and bentonites and plenty of zircopax. And that it is possible to dip and paint engobes on bisque if the slurry is conditioned with gum. This recipe looks so good because it promises to enable the strength of terra cotta with the whiteness of porcelain. This in turn promises to provide white surfaces for brush work and transparent overglazes that produce brilliant gloss and colors not achievable at high temperatures.

It appears that to be paintable on porous bisque a low specific gravity is required, that means lots of water, almost double the normal amount. How to gel that watery slurry into a creamy consistency? 3% Veegum! To make it bond well with the body CMC gum is needed. These gums can of course be tuned to thicken or thin the slurry and make it dry faster or slower. The slurry should thicken overnight after initial mixing.

Having this high water content means that it is not possible to dip-glaze thin-walled ware inside and out without water-logging it. I have found that pour-application on the insides of pieces works extremely well, the engobe drains to a perfectly even thickness and drip dries enough to stop runs in about a minute.

Of course, engobes do not bond with the body as well as glazes, so this recipe may need further adjustment to the firing shrinkage to match the body at cone 04. Interestingly, another option is to fire lower or higher, where the match might be better (I am working on trying this at cone 05, 04, 03, 02 and 01). For higher firing shrinkage I will add 1-2% frit 3110. Making the firing shrinkage lower will be harder, but that is not likely since the terra cotta body is almost certain to be the one that has a higher firing shrinkage.

I mixed this 900:500 water:powder to get about 65% H2O.

Pictures

L3685Z1 and L3685Z2 at cone 04

Z2, this improved version, is much whiter. On Plainsman L210. Fit is amazing for Z2. And it is incredibly white.

L4170B with L3685Z2 engobe

With the gum additions, it applied very evenly to the bisque (using a pour-in pour-our technique). The layer was very thin yet covers very well.

Cone 04 L3685Z2 white engobe with clear glazes

The colour as amazingly white. And this engobe appears to be fitting very well even though we are dipping pieces, like a glaze, to bisque ware.

L215 with L3685Z2 engobe at cone 04

Although a very thin layer it completely covers the dark red body.

On L215 cone 03+ shivers on rim

Cone 1-6 (bottom to top), cone 01, 02 at top

Cone 03-04 Expansion Adjustable Gloss Base

Total:102.00

Auto Unity Formula + Analysis

Notes

*This recipe provides the greatest thermal expansion adjustability we have seen in a low fire clear glaze. It combines a middle-of-the-road thermal expansion frit with a very low and very high expansion frit (they cancel each other out but increase gloss of the otherwise silky Frit 3195).

Do not assume the glaze fits because a piece emerges from the kiln without crazing or shivering. Thermally stress it (by two minute boiling-water and ice-water immersion cycles). We refer to this as the 65:10:10:15 recipe.
If this crazes, try 65:20:15 (3195:3249:EPK).
If it shivers, use 65:20:15 (3195:3110:EPK).

The ball clay also imparts good working properties and it has a lower LOI than EPK (to generate less micro-bubbles in the glaze).

We found about 3800 water for 4000 water to get 1.44SG. The slurry gels more than with EPK.

The iron oxide is a fining agent to remove the microbubbles. It also enhances the red color of terra cotta (when used as a transparent). If you are adding stains or using it on a white burning body, remove the iron.

We always firing pieces using a drop-and-soak firing schedule. Program your kiln to make the find approach to cone at 100F/hr, hold there for ten minutes, drop 100F quickly and hold there for 30 minutes.

Pictures

G1916Q at Cone 01 on 3D+iron

Very nice results on L3724E red body at cone 01. Piece is very strong.

1916J and Q fired to 1450F

These glazes are starting to melt, the surface having reached the consistency of a porcelain and have densitfied to very low porosity. Notice the iron in the ball clay really shows up at this stage (it will be less evident later).

G1916Q Cone 04 using 04DSSC schedule

Counterclockwise: L212, Raku, Buffstone, L213, L210, L215 Crazing out-of-the-kiln on Raku and buffstone. Bisque 04-03.

G1916Q and J fired 1650-2000F

Ten-gram balls melted and flattened as they fired. They soften over a wide range, starting well below cone 010! At 1650F carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon; think of what a raw glaze might have! Of course, this is a thick layer, so the bubbles are expected. But they still can be an issue on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (1450F in this case), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible.

Success with cyrstal clear glaze cone 03

Uses Cone 03 soak-soak-slow cool schedule. Left: P6282 with 3685U slip and 1916Q. Clear and very good. Glaze is thicker than the other two. Shivering on lip, the slip is not fitting the body. Center: P6282 with G2931F Ulexite clear. Better clarity even though it is applied very thin. Shivering on lip, the slip is not fitting the body. Right: L3724F with 3685U slip. No shivering. Very good coverage of the glaze, very clear, the best I have seen yet!

G1916Q on L215, L212, L210, L213, Buffstone at cone 03

All exited from the klin without crazing. The L215, L213, L210 and L212 samples subsequently survived a 300F/Icewater test without crazing, but the Buffstone did not. The L213 would not likely survive a cold-to-hot test without shivering.

1916Q cone 04, 03

Both were slow cooled. While the cone 04 version is glassy and ultra-gloss, it has significant clouding of micro-bubbles. The cone 03 version, right, is completely transparent.

G1916Q on L210 fired at cone 04

G1916Q+2%Iron on L212 talc body fired at cone 05

This will likely shiver over time. But the speckle that happens on white bodies is clearly visible.