• Materials for Ceramics

Redart

Alternate Names: Red Art

Description: Red Firing Earthenware

Oxide	Analysis	Formula	Tolerance
CaO	0.20%	0.02
K2O	4.10%	0.27
MgO	1.60%	0.25
Na2O	0.40%	0.04
TiO2	1.10%	0.09
Al2O3	16.40%	1.00
SiO2	64.20%	6.64
Fe2O3	7.00%	0.27
LOI	4.90%	n/a
Oxide Weight		590.85
Formula Weight		621.30

Notes

July 2026: Reports are surfacing that the material is now less plastic and burns with less color and maturity. If you use it as an ingredient in stoneware bodies, then test adjustments to compensate. Add iron oxide to help with color, feldspar to restore fired maturity and bentonite to restore lost plasticity. The SHAB test is invaluable to do the physical testing and side-by-side comparisons to zero in on the recipe changes needed. Low fire bodies don’t vitrify anyway, so color and plasticity are the concerns.

Jan 2022: We got reports of Redart properties possibly showing some variation; we arranged testing on separate shipments arriving at different places and spanning a year. The results for drying shrinkage, water of plasticity, drying performance and fired shrinkage were very similar for all.

Redart is a red-firing earthenware of moderate to low plasticity and low shrinkage. It fires light orange to dark red depending on firing temperature from cone 06 to 3. It fires much stronger and denser at cone 04 than 06 and achieves its best color/strength compromise at cone 02. It produces deep red color and stoneware properties at cone 1. It is air-floated to 200 mesh and has a long firing range. This is a very popular ingredient in clay bodies made across North America and it has been available for many years.

This material is quite high in iron thus the powder is a deep red color and quite messy to work with. Redart is also used by potters in glaze recipes, sometimes up to 60%.

Redart can be used as a primary component in brown and red-burning modelling and low-temperature throwing bodies. Since Redart is not plastic it needs help from bentonite ball clay and bentonite. The L4170BP recipe is a good example.

Redart can be used to make casting bodies without any other additions. It will produce very good red color and will deflocculate and cast well. It is possible that a small addition of plastic clay might be needed to give it better strength to pull away from the mold. Also, additions of silica (for glaze fit) and a frit (for maturity) are also options. The L4170B recipe is a good starting point, it is 90% Redart.

While Redart can be used as a source of iron in high-temperature bodies, it will produce brown rather than red coloration. This is because the fluxes are intimately mixed with the iron and fuse it to a darker color. To get red in oxidation or reduction at higher temperatures, you must use refractory clays with iron or iron-bearing fireclays. However, there is good reason not to use low-fire reds as fluxes in high-fire bodies: The fluxes in the red clays don't dissolve cristobalite as feldspar does; the result can be thermal expansion related ware failure.

This material is mined from a large deposit and the company is confident in its consistency and long term availability.

Related Information

Redart 2010 test bars:

As a physical properties reference

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Here is what Redart fired like in 2010 when evaluated for physical properties using the SHAB test and LDW test. These bars are fired from cone 5 down to 06 (top to bottom). It reached zero porosity between cone 1 and 2. Beyond that it was stable to cone 4. Its drying shrinkage was only 5%, indicating low plasticity (compared to a typical pottery clay), but plenty for casting. It has no soluble salts and a perfect score on my drying performance DFAC test. Cone 04 and 06 porosity and fired color are very typical of terracotta clays.

Redart has a published chemical analysis:

But it doesn’t indicate what you might think

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This slurry is 100% traditional Redart. This deep red color is a product of the high percentage of iron oxide in its chemical analysis. But that chemistry provides a good example of why using the classical ceramic rational-analysis method to derive a theoretical mineralogy can be misleading for an illitic clay (which this is).

The traditional calculation assumes that K₂O and Na₂O belong to feldspars, then assigns the remaining Al₂O₃ to kaolinite and the remaining SiO₂ to free quartz. However, Redart is known to be an illitic sedimentary clay. And illite itself contains substantial K₂O. That 4.1% K₂O is a strong chemical confirmation of Redart containing a lot of illite/mica-type clay. The 1.6% MgO is another clue that some of the chemistry belongs to smectite minerals rather than discrete feldspar and MgO phases.

Redart fired bars vs. Plainsman Blue Grey Plastic

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The Redart clay bars (left) are fired at cones 06, 04, 2, 4 & 5 (top to bottom). The Plainsman Blue Grey Plastic bars (right) are fired at 06, 04, 03, 02, 2 & 4. The SHAB test procedure (used to make these) gives us the firing shrinkage and porosity at each temperature, these are direct indicators of the fired maturity. And it enables comparing the fired color and surface character. Notice how much the color changes with increasing temperature. The fired maturity is pretty similar but the BGP is a little browner. It is also much more plastic (the drying shrinkage is significantly higher).

Redart (left) vs. Lizella clay. Definitely not substitutes for each other.

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These bars have been fired at cones 4, 2, 02, 04 (top to bottom) using the SHAB testing procedure. We can measure fired shrinkage and porosity in each to get an indication of their fired maturity. The Redart (left) is much more vitreous and reaches almost zero porosity by cone 4 whereas the Lizella still has 11% porosity at cone 4. Lizella also has a much higher drying shrinkage (because it is way more plastic).

Redart-as-a-glaze test piece

Wood-fired on Laguna B-Mix

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Fired to cone 13 in a Manabigama wood fired kiln.

Melt fluidity and coverage:

RedArt Slip vs. Albany Slip vs. Alberta Slip

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These three melt flows and mugs were fired at cone 6 (using the C6DHSC firing schedule). The benchmark recipe is 80% clay and 20% Ferro Frit 3195 (the standard GA6-B recipe).
-The center melt flow (and matching buff stoneware mug below) employ the original Albany Slip.
-The one on the right employs Alberta Slip. Notice that, although having a very similar melt flow, it needs an iron oxide addition to darken the color (e.g. 2%).
-The one on the far left uses an Albany Slip substitute made from 80% Redart, 6.5% calcium carbonate, 6.5% dolomite and 6.5% nepheline syenite (code L3613D). The chemistry of RedArt is different enough from Albany that some compromises were needed to avoid over-supplying the iron even more (and firing darker yet). Although this Redart version runs in a very similar pattern on the melt flow, the character of the glaze on the mug reveals it needs a little more melting (increasing the frit percentage would take care of that).

A highly grogged body that is almost porcelain-vitreous

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This is an unusual cone 10R vitreous sculpture clay (code number L3118J). The grogless base burns dark brown and is almost porcelain-vitreous. All of the surface variegation is a product of the contrasting colour of the dark brown body and lighter coloured grog. Almost all of the fired porosity is from the refractory grog particles.

This was inspired by Craft Crank, a UK-made highly grogged body: A super-plastic base that can host a high percentage of coarse grog particles yet still offer very good workability (and be throwable, even non-abrasive, on the wheel). My base has 40% ball clay, 10% plastic kaolin, 20% Redart, 5% quartz and 25% 20x48 grog. Redart is an illitic clay, containing K₂O like a feldspar, the 20% here has enough fluxing power to vitrify the body. Given recent changes in Redart, this recipe could be modified to compensate (e.g. add iron oxide, use 45 ball clay instead of a 50 ball clay/kaolin mix, add 5% feldspar).

A heavily grogged body:

Yet throws surprisingly well

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This cone 10R piece is about 8 inches tall and quite thin-walled. This is L3118J grogged clay with 25% Christy Minerals STKO22S grog. It is not a typical grog having a range of particle sizes, these particles are all close to 20 mesh. Because the base body, mainly Redart and Ball clay, is so fine and plastic, and the grog is only one size, this is not only throwable but resistant enough to splitting that large pieces can be made. The plasticity of the base and the mobility of the grog particles within enables applying pressure with surprisingly less abrasion than expected. And of course, the grog makes it highly resistant to cracking during drying.

Body frit makes Redart a stoneware

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Stoneware at cone 02? Yes. These test bars are fired to cone 02. The top body is 50:50 Redart and a silty raw material from Plainsman Clays (named 3D) plus some bentonite and 1% iron. The bottom one also has 5% Ferro Frit 3110. The porosity: The bottom one is 3%, the top one 8%. So each 1% frit reduces the porosity by 1% in this case.

M2 vs. Redart

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Plainsman M2 (left) vs. Redart (right). These bars are fired cone 04, 02, 2, 3, 4 (top to bottom). Fired color is almost identical. M2 has a little more soluble salts (however M2 has no chemical additions to precipitate them, it is ground as the pure mined material). M2 is more plastic (although still not as plastic as a typical pottery clay). However, Redart will make a good casting slip while M2 does not respond to deflocculants.

A Redart cone 03 body shines when it come to ease of glaze fit

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These bowls are fired at cone 03. They are made from 80 Redart, 20 Ball clay. The glazes are (left to right) G1916J (Frit 3195 85, EPK 15), G191Q (Frit 3195 65, Frit 3110 20, EPK 15) and G1916T (Frit 3195 65, Frit 3249 20, EPK 15). The latter is the most transparent and brilliant, even though that frit has high MgO. The center one has a higher expansion (because of the Frit 3110) and the right one a lower expansion (because of the Frit 3249). Yet all of them survived a 300F to icewater IWCT test without crazing. This is a testament to the utility of Redart at low temperatures. A white body done at the same time crazed the left two.

Brown soluble salts that appear after drying, but disappear on firing

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The soluble salts dissolved in the water of plasticity of this red body have migrated through the white engobe during drying of these earthenware cups. The cups were upside down so all the solubles have been left on the outside surface. The red body is made using a high percentage of Redart clay (a widely available commercial low-fire low-plastic clay in North America). It is plasticized using added ball clay. The brownish material is organic, because after bisque firing it has disappeared.

Original container bag of Redart

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Back side of Redart original container bag

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Links

Articles	Understanding the Deflocculation Process in Slip Casting Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.
Articles	Understanding the Terra Cotta Slip Casting Recipes In North America This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.
URLs	https://www.rescoproducts.com/docs/default-source/data-sheets-us/pds/redart-clay.pdf Resco Clays Data Sheet
Materials	Goldart
Materials	Ceramic Fireclay
Materials	Roseville Clay
Materials	Salt Lick
Materials	Carbondale Clay
Materials	Newman Red Clay
Materials	Redearth
Materials	Banta Red Clay
Materials	Lizella Clay
Typecodes	Clay Other Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.

Data

Drying Shrinkage	5.5%
Firing Shrinkage	Cone 04: 3% Cone 2: 8%
Water absorption	Cone 04: 11% Cone 2: 1%

Mechanisms

Body Maturity	Red terra cotta clays are typically 6-8 cones more vitreous than is possible with feldspar-white clay mixes. Thus where a red clay can be tolerated, materials like this can be employed to create cone 02-2 stoneware.

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