Cone 04 Terra Cotta Casting Body and Transparent Glaze

Share from (Lab Documentation and Calculation System) by Digitalfire

Terra Cotta is often associated with indigenous cultures or school art programs. But actually, a quality terra cotta with a good glaze can produce near stoneware durability.

Project Name

A new idea for production at cone 04


Terra Cotta is being discovered by many potters (and hobbyists with interest in production) as being alot 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 so much brighter than at high temperatures. It seems there are thousands of commercial glaze products for use at cone 06-04.

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.

Cone 04 UltraClear Glossy Base

Code #


Materials Amt
Fusion Frit F-524 850.000 82.93%
Fusion Frit F-69 40.000 3.90%
EPK 90.000 8.78%
Silica 45.000 4.39%


Auto Unity Formula

BaO 0.04
CaO 0.44
MgO 0.05
K2O 0.10
Na2O 0.12
(KNaO) 0.21
SrO 0.25
B2O3 0.66
Al2O3 0.47
SiO2 3.91


Si:Al: 8.4:1
SiB:Al: 9.8:1
R2O:RO: 0.2:0.8






11.06 per kg


I developed this for cone 04 to fit as many clay bodies (without crazing) as possible (my Zero3 clear works well on specific bodies but does not melt enough at cone 04 and it's thermal expansion is too high to fit some bodies). I am developing a recipe for a terracotta casting body at the same time as this and I am working to make it compatible also.

I found the inspiration for this on testing many commercial clears and finding one that stood above the others regarding fit and clarity at 04. I have always been under the impression that low fire bodies have a wide enough range of thermal expansions that one glaze cannot be expected to fit them all. And that if a glaze melts well at cone 04 it will have too much melt fluidity past cone 02. But this glaze has made me question both, I am shocked at how it is possible that it can fit so many bodies and work across such a wide temperature range. In fact, I have not found a body that it does not fit! So I had it analyzed at a lab and then created a recipe to source its chemistry. A stroke-of-luck was that a frit we already use to produce another glaze, Fusion Frit F-524, is close to the complete chemistry needed.

I have been surprised by other aspects of this recipe. It is crystal-clear on any body at any thickness (at cone 04 and above)! Additions of tin and zircon produce a white that melts just as well as the clear. It is amazing how the mobile the melt is, how it runs right off my fluidity checker! Yet it is not significent more mobile at cone 1 than cone 04. There is some kind of magic with this chemistry that I am anxious to learn more about.

While it hardens to a powdery surface, it is amazing how little gum solution is required to make it dry hard and hang on to the bisque when other layers are added on top.

For the first mix I added 3000g of powder to 2400g of water to produce a specific gravity of 1.48SG. This produced a watery slurry. I added epsom salts to the point where further additions did not thicken the slurry (up to a total of 7g). This improved it considerably but it was still a little thin (although it covered and applied like a typical dipping glaze, drying in seconds on bisque ware). However the surface was too powdery so I removed 400g of water and replaced it with 400g gum solution. This slowed dip time to about a minute (waiting for the dripping to stop) but it now tolerates thick overlaying of Majolica colors (without them pulling it away from the bisque). For a single-coat dipping (where no overglaze work will be done) I would use about half the amount of gum solution.



Mike ODonnell and Fusion Frits says many customers use F280 and F38. He suggested F5 might be most similar to this. But I found that F524 was by far the closest.


G3879 on Plainsman L210, L215 at cone 04

These are 42 mesh low fire bodies. They normally have issues with pinholing but using this glaze the results are stunning. The L210 contains no talc, the L215 has 10%, yet this glaze does not craze on either one (over time it shivers on the L215).

Melt fluidity comparison with #1 commercial clear

We tested half-a-dozen commercial clears and found G3859 to be the best all-around one. This one has a very similar melt fluidity.

G3879 Clear glaze on Plainsman L211 - Cone 04

Glossy, crystal clear, no crazing! And this is a 42 mesh body containing zero talc.

G3878 has a high surface tension

As can be seen in the way it has melted here.

G3879 on SIAL 25F, Plainsman J2, L4170 TerraCotta

These are very different bodies. The leftmost contains talc to raise the thermal expanison to help prevent crazing with commercial glazes. The center one contains nepheline syenite (for the same purpose). The terra cotta on the right is just Redart and ball clay. This glaze fits are all three!

Sial 10F, 25F with G3879 clear glaze at cone 03

The glaze is applied double-thickness on the top half. Yet there are no more bubbles or crazing. Perfect fit.

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.

G3879 with 5% Tin Oxide on SIAL 10F

When mixing Tin (as an opacifer), it is very important to mix it well. The one on the left was mixed poorly (at high speed with my propeller mixer but not for long enough). The one on the right was mixed much better and so produces better opacity. Tin is expensive so this is important.

G3879 on Plainsman Buffstone - cone 04

Buffstone is an entry-level low-price body not intended to fit commercial glazes. Yet the glaze fits! And without any surface defects.

G3879 Zircon White on SIAL 25F, 10F - cone 03

10% zircopax has been added. It is melting well so the percentage could be increased for great opacity on red burning bodies.

G3879 at cone 1 on SIAL 10F, 25F

Crystal clear, no running. Perfect!

GBMF test on G3879 at cone 1

It is not running and flowing nearly as much as expected. The melt surface tension holds it in place, so it should be able to fire to cone 2 and beyond.

G3879 on terra cotta at cone 04, 02, 1

This is on the L4170 body, it is a lighter firing product, 25F, from SIAL.

G1916M, G3879, G2931K on L215 - Thick

Thickly applied encourage poor fit to show up.
Clearly, as shivering and cracking demonstrate, G3879 is under excessive compressive on L215.
The other two are not showing any issues (other than heavy bubbling because of the thickness).

G1216M is a blend of 3124/3124 with kaolin. G2931K is the Zero3 clear.

L213 with G3879 glaze at cone 04

Survived 325F:IceWater test with almost no crazing. However there was a little shivering on the rim after a month. Another mug had no crazing on the inside after several months.

G3879 on L4115J2 buff body at cone 04

Glaze is ultra clear.
It was refired at cone 022 to apply a decal.
It was 325F to icewater tested without crazing.
It was waterlogged (the bottom is bare clay) and then put in a microwave for two minutes. Despite getting incredibly hot it did not fracture or craze!

G3879 on SIAL 10F at cone 02

No crazing after several months. Flawless service.


B - Tin White

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Cone 04 UltraClear Glossy Base" id="154451" key="zpjbwJ14" date="2019-10-03" codenum="G3879"> <recipelines> <recipeline material="Fusion Frit F-524" amount="850.000"/> <recipeline material="Fusion Frit F-69" amount="40.000"/> <recipeline material="EPK" amount="90.000"/> <recipeline material="Silica" amount="45.000"/> </recipelines> </recipe> </recipes>

Born: 2019-04-15, Modified: 2019-10-03 11:13:07

Terra Cotta Casting - Cone 04

Code #


Materials Amt
Redart 80.000
*KT1-4 Ball Clay 20.000
Additions Amt
Water 44.000
*Darvan #811 0.960



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 ball clay is not slowing down casting, yet adds so much to the leather hard handling strength and dry strength.

So far, this has proven to work perfectly with my G3879 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.

I am going to work on fitting an adapted version of my white engobe to this. I will apply it to the outsides of cast pieces by filling the mould with the deflocculated white engobe first and then pouring it out after a few seconds. Then I’ll fill the mould with the terracotta slip. That will produce a white burning body with the stoneware-like fired strength of a terracotta at cone 02! Next will be a fire-engine-red version of the G3879 clear glaze.

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. 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.


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


SHAB - Shrinkage/Absorption

1 94.38 93.64 48 55.07 -6.0 5.6% 0.8% 14.7%
2 94.38 89.9 48.34 51.45 -2.0 5.6% 4.7% 6.4%
3 94.3 91.29 48.78 53.53 -4.0 5.7% 3.2% 9.7%
4 94.46 89.45 47.7 50.12 -2.0 5.5% 5.3% 5.1%
5 94.34 89 47.29 49.53 1.0 5.7% 5.7% 4.7%
6 94.42 88.73 48.05 49.84 2.2 5.6% 6.0% 3.7%

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Terra Cotta Casting - Cone 04" id="155277" key="HNJuTMr7" date="2019-05-17" codenum="L4170"> <recipelines> <recipeline material="Redart" amount="80.000"/> <recipeline material="KT1-4 Ball Clay" amount="20.000"/> <recipeline material="Water" amount="44.000" added="true"/> <recipeline material="Darvan #811" amount="0.960" added="true"/> </recipelines> </recipe> </recipes>

Born: 2019-03-20, Modified: 2019-05-17 10:06:49

Firing Schedule Name

Plainsman Lab Test Firing Cone 04

Finish by falling cone

Degrees (Fahrenheit or Celcius)


Schedule Type


Start Time and Temperature

8:30 am (no temperature specified)


Step Degrees/Hr Temperature
Time Note
1 300 220 60 1:44 10:14 am
2 400 1822 5:44 2:14 pm
3 108 1922 10 6:49 3:19 pm


ConeArt Niv 2018: 1922
Autofire: 1915 good - Oct 4, 2016