Replacing talc with nepheline syenite and dolomite in clay bodies

Share from Insight-live.com (Lab Documentation and Calculation System) by Digitalfire.

See Also:
Cone 04 Terra Cotta Casting/Throwing Body, Transparent Glaze, Firing Schedule
Our new ultra-clear cone 04 transparent base glazes for 2021
Five Low Fire Base Clear Glazes: What is the Best Strategy?

As a starting point, we are transitioning a standard cone 04 50:50 talc:ball clay body to a nepheline/dolomite/ball clay alternative mix. And we have two base glazes that fit.


Project Name

Replace talc in low temperature white body

Project Codenumber

UnAssigned

Notes

Talc has always been the "accepted" way of making low fire white bodies. Mixtures of ball clay and talc produce the outstanding working, casting, drying and firing properties that make this usable by hobbyists. Talc is the secret behind raising the thermal expansion to fit commercial cone 06-02 glazes. And this mix creates fantastic plastic working properties (and casting). Until now, in North America, talc and ball clay grades could be selected to produce a very white burning body. Although the traditional talc mix is highly porous and has poor strength in the fired state, these factors are conveniently ignored in order to provide hobbyists and artware producers a white "canvas" on which to apply colorful glazes! That means the dark-burning talcs are "non-starters".

However a major problem arose during 2021: Texas talc ceased production. North American industry initially looked toward other talcs, but they fire significantly darker or are also ceasing production. A climate of litigation seems to be the reason for the demise of this material. A Chinese talc is available, but unlike the Texas talcs, it is not asbestos-free. Fortunately it is possible to make a body that fires just as white and strong with zero-talc.

After a series of tests we have settled on a 30:20 Nepheline Syenite:Dolomite mix to substitute for the 50% talc in our main recipe, L213 (Plainsman made a test run of this Aug 2021). Based on the results of this we will do likewise in other talc-containing bodies. Other suppliers are working on this are trying wollastonite instead of dolomite and silica as a filler instead of nepheline. We are avoiding these, both have safety implications: quartz particles in the silica and mineral fibres in the wollastonite. We believe our material selection will produce a body that is safer-to-use than anything ever made up until now. And all three materials are easily available (nepheline syenite is a Canadian material, that is good for us, if you are in the US consider using a white feldspar available in your area).

The main problem is going to be glaze fit. We have transparent glazes that fit it well, they fire crystal clear. But commercial glazes from companies like Mayco, Duncan, Spectrum, AMACO are crazing (either out-of-the-kiln or later). This being said, we are working to discover a firing schedule that might activate the cristobalite-generating potential in the dolomite (this is possible because it is high in MgO like talc).

Actually, this talc-demise problem is so severe that industry-wide changes may be needed. That means glaze manufacturers are going to need to reduce the thermal expansion of their glazes, using base recipes similar to the two shown here. These fire to a better ultra-clear than any of the 5 base commercial clear glazes we have tested. Stains and opacifiers can be added to these to make any color or effect. For now, consider substituting one of these as your transparent, and continue to use all the commercial colored glazes (and tolerate possible crazing with them). Another factor: World-wide supply chain issues could affect availability of glazes you use, so knowing how to mix your own from scratch and adjust it to be a brushing glaze, dipping glaze or base coat, is not a bad thing! The proportions of water:powder that we use are shown. Go to https://digitalfire.com/article/98 for more information on getting started mixing your own.

If you are in production, you might want to consider using a terra cotta, for example our G4170B (plastic and casting versions both work very well). For a white surface use the matching engobe, L3685Z2. See the link above for more information. Terra cotta bodies fire to much better strength and glaze fit is a non-issue (everything fits). Terra cotta bodies are also less expensive and have excellent workability. And, by firing to cone 03-02 stoneware strength can be achieved.

Low Fire White NS/Dolomite Body

Code #

L4410K

Materials Amt
*KT1-4 Ball Clay 50.000
Nepheline Syenite 35.000
Dolocron 4013 (Dolomite) 15.000
Additions Amt
Bentonite 3.000

Total:103.00

Notes

*L213 is a body made by Plainsman Clays, it has traditionally employed Texas talc and KT1-4 ball clay, a simple 50:50 mix. This is part of a project to explore the possibility that a Nepheline Syenite/Dolomite blend could replace it.

This is the plastic version of the body, for use on a potters wheel. For casting, reduce the bentonite to 1%. If casting is too slow remove it completely (if ware is then unable to pull itself away from the mold, restore about 0.5% bentonite).

Talc is is a magnesia mineral and so is dolomite. Can a small amount of dolomite encourage the growth of cristobalite and increase the thermal expansion of a body the same way that talc does? This recipe appears to say yes!

Initial glaze testing of this is looking very promising. Fired whiteness is amazing, quite a bit whiter than our former L213 talc:ball clay mix. We are stunned by the quality of pieces we have been able to produce with this using standard low fire Spectrum clear glazes. Used in functional ware it appears crazing will eventually occur in ware fired at cone 04 (using Spectrum glazes).

Plasticity and workability is also as good or better than the former talc body.

A real plus is that this body employs Nepheline Syenite as a filler, instead of quartz. That makes it even safer to use.

Our G3879C and G1916QL1 glaze recipes are working well on this. That means the suppliers and users can make these as dipping glazes for functional surfaces (or as bases for additions of color and opacifiers).

Pictures

L4410K with G1916Q, G3879, G3879#2, G1916QL cone 04

Without looking at the base any of these could be mistaken for a high fire porcelain with clear glaze! After several months there is no sign of crazing. G3879 was slow-heated and slow-cooled through cristobalite inversion during bisque.

L4410K cone 03 with spectrum brush-on glazes

Spectrum 742 dark red, 705 black, 700 clear. Out of the kiln there was no crazing but over a few weeks they did.

L4410K - After several months these were not crazed

For some reason, it appears that cast pieces are crazing and less. The crystal clear glossy surface is a stunning quality. These had a Cristobalite firing schedule bisque.

Spectrum 705 Black, G1916QL clear on L4410K at cone 03

The quality of this piece is amazing! The evenness of coverage that can be achieved with brush-on glazes is impressive. This was my favourite coffee mug. However it did finally craze after about 2 weeks of use (that motivated the recipe change to G1916QL1).

By cone 6 L4410K is bloating and warping

L4410K with G3879G1 and G1916QL at cone 04

Both are crystal clear, results are stunning! Both the G3879G1 and G1916QL glazes held up well in my kitchen at home. For a while. The G1916QL on crazed after a couple of weeks. The G1 crazed after about a month.

L4410K cone 04 Spectrum reds

Left: 742 Dark Red, right 748 Fire Engine Red Insides: G1916QL, G3879B right (the latter is significantly whiter because it uses a kaolin rather than a ball clay) The 1916QL began to spiral craze on the inside after a couple of weeks using in our kitchen.

Untitled

L4410K with Spectrum 700/746 glazes

No crazing after two months.

Testdata

SHAB - Shrinkage/Absorption

DLEN FLEN FWT BWT CONE DSHR FSHR ABS
1 93.72 93.17 37.16 43.7 -6.0 6.3% 0.6% 17.6%
2 93.63 92.82 37.66 43.99 -4.0 6.4% 0.9% 16.8%
3 93.6 92.7 36.58 42.52 -3.0 6.4% 1.0% 16.2%
4 93.65 92.66 36.42 42.61 -2.0 6.3% 1.1% 17.0%
5 93.69 92.81 36.09 41.83 -.8 6.3% 0.9% 15.9%
6 93.74 92.84 36.44 42.35 1.0 6.3% 1.0% 16.2%
7 93.78 92.71 36.06 41.79 2.0 6.2% 1.1% 15.9%
8 93.6 92.52 36.91 42.59 3.0 6.4% 1.2% 15.4%
9 93.64 92.29 35.52 40.1 4.0 6.4% 1.4% 12.9%
10 93.55 91.62 35.57 38.04 5.0 6.5% 2.1% 6.9%
11 93.6 90.97 33.71 33.77 6.0 6.4% 2.8% 0.2%

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Low Fire White NS/Dolomite Body" id="198855" key="GvvNLtja" date="2021-10-07" codenum="L4410K"> <recipelines> <recipeline material="KT1-4 Ball Clay" amount="50.000" tolerance=""/> <recipeline material="Nepheline Syenite" amount="35.000" tolerance=""/> <recipeline material="Dolocron 4013" lookup="Dolomite" amount="15.000" tolerance=""/> <recipeline material="Bentonite" amount="3.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2021-02-06, Modified: 2021-10-07 12:50:14

Low Fire White NS/Dolomite Body 2

Code #

L4410L

Materials Amt Units
*KT1-4 Ball Clay 50.000 GM
Nepheline Syenite 30.000 GM
Dolocron 4013 (Dolomite) 20.000 GM
Additions Amt Units
Bentonite 3.000 GM

Total:103.00 (R)

Notes

*This increases the dolomite from 15 to 20% (at the expense of nepheline syenite). The L4410K did craze the glazes over time. A test run of this body was made at Plainsman Clays August 2021.

This is the plastic version of the body, for use on a potters wheel. For casting, reduce the bentonite to 1%. If casting is too slow remove it completely (if ware is then unable to pull itself away from the mold, restore about 0.5% bentonite).

Unless otherwise noted we generally use our C04PLTP firing schedule.

Some dry strength has been lost (compared to L4410K).

Pictures

Spectrum 700 clear and 742 red on L4410L

Out-of-the kiln there is no crazing, results are stunning!

K4410K (left), L4410L (right) - Cone 6

This body has another advantage over the talc version: Although it begins to warp at cone 6 (stoneware temperature), it is not brittle, fired strength is excellent. That means that it should survive to cone 4. Another things that the is remains porous if bisque fired to cone 02 or even 2. This

Testdata

SHAB - Shrinkage/Absorption

DLEN FLEN FWT BWT CONE DSHR FSHR ABS
1 93.77 93.46 34.01 40.5 1.0 6.2% 0.3% 19.1%
2 93.67 93.32 35.87 42.23 2.0 6.3% 0.4% 17.7%
3 93.38 92.95 35.78 42.23 3.0 6.6% 0.5% 18.0%
4 93.01 92.51 35.89 41.72 4.0 7.0% 0.5% 16.2%
5 93.18 92.76 35.51 40.3 5.0 6.8% 0.5% 13.5%
6 93.42 91.14 35.41 35.51 6.0 6.6% 2.4% 0.3%
7 93.38 93.21 35.81 6.0 6.6% 0.2%
8 93.44 93.1 35.27 42.02 -4.0 6.6% 0.4% 19.1%
9 93.61 93.37 35.43 42.01 -1.8 6.4% 0.3% 18.6%
10 93.83 93.54 34.93 41.36 -.8 6.2% 0.3% 18.4%

SOLU - Soluble Salts

FIRE
1 nil

DFAC - Drying Factor

DFAC SOLD
1 A000 vlight

CLWC - Clay Water Content - Powder, Plastic

PUGW PUGD POW% PUG%
1 25.74 20.48 20.4%

LDW - LOI/Density/Water Content

WWGT DWGT FWGT H2O LOI DENS
1 13.03 10.56 8.97 19.0% 15.1%

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Low Fire White NS/Dolomite Body 2" id="201792" key="XBCXq7fe" date="2021-10-07" codenum="L4410L"> <recipelines> <recipeline material="KT1-4 Ball Clay" amount="50.000" tolerance="" unitabbr="GM"/> <recipeline material="Nepheline Syenite" amount="30.000" tolerance="" unitabbr="GM"/> <recipeline material="Dolocron 4013" lookup="Dolomite" amount="20.000" tolerance="" unitabbr="GM"/> <recipeline material="Bentonite" amount="3.000" unitabbr="GM" added="true"/> </recipelines> </recipe> </recipes>

Born: 2021-02-06, Modified: 2021-10-07 12:50:34

Cone 05+ Lower Expansion glaze

Code #

G1916QL1

Materials Amt
Ferro Frit 3195 60.000
Ferro Frit 3249 25.000
New Zealand or Grolleg kaolin (Grolleg Kaolin) 10.000
45 micron Silica (Silica) 5.000

Total:100.00

Auto Unity Formula

CaO 0.51
MgO 0.28
Na2O 0.20
(KNaO) 0.20
B2O3 1.10
Al2O3 0.51
SiO2 3.03

Ratios

Si:Al: 5.9:1
SiB:Al: 8.0:1
R2O:RO: 0.2:0.8

Expansion

6.0 (Molar:6.0)

LOI

1.2

Notes

*This is a much-lower-thermal-expansion version of our standard G1916Q low fire clear recipe (which was delay-crazing on the L4410C, L4410K, L4410L cone 04 test bodies). The lower expansion was achieved by eliminating all high-expansion 3110 and more than doubling the low-expansion 3249. This now shivers on our L4170B terra cotta (use the G1916Q for it).

This also switches the ball clay for New Zealand kaolin, producing a much more transparent glass. And this is melting better (since frit 3249 is a very active flux).

The lower surface tension of this make it a better candidate than G3879 for opacification with zircon.

1500g powder, 1200g water produced a specific gravity of 1.49 and yield just over 1.80 litres. Sieving to 80 mesh was required to break down the kaolin agglomerates.

Comments

2021-09-11

Joe: August 12/21
1500 grams powder
1200 grams water.
Screened 80 mesh. S.G. 1.49
At this S.G. yield is just over 1.80 litres.
Glaze seems a little thick, maximum 5 second vigorous stir time to stettling after allowing to saturate overnight. Glaze falls off spatula very easily so am leaving here for Tony to evaluate. This glaze was difficult to screen at this S.G. but any more water and it might be too thin.

Joe: I am thinking if you want to properly assess glazes with New Zealand Kaolin in them that you have to screen the glazes first. This kaolin does not break down easily and you can see tiny dots of clay on the spatula when you stir glaze even after it is allowed to saturate overnight. Once screened 80 mesh, these agglomerations disappear.

Pictures

G3879C, G1916Q, G1916QL1 on L213 Cone 04

The move from ball clay to New Zealand kaolin (centre mug vs right mug) greatly improves the clarity (because of the reduction in iron and titanium).

Cone 04 G1916QL1 vs G3879C melt flow comparison

G1916QL1 is more melt fluid so it is likely useful before cone 04. But G3879C is also very fluid (it also has a higher surface tension). Both are producing textbook high quality flows.

Spectrum 705 Black, G1916QL clear on L4410K at cone 03

The quality of this piece is amazing! The evenness of coverage that can be achieved with brush-on glazes is impressive. This was my favourite coffee mug. However it did finally craze after about 2 weeks of use (that motivated the recipe change to G1916QL1).

G3879C, G1916QL1 on L4410L Bisque 1800 glaze cone 04

No crazing after weeks of use, ultragloss. Frit 524 has much less iron, that is why that mug is so much whiter.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Cone 05+ Lower Expansion glaze" id="202545" key="pCPjG9Jo" date="2021-10-01" codenum="G1916QL1"> <recipelines> <recipeline material="Ferro Frit 3195" amount="60.000" tolerance=""/> <recipeline material="Ferro Frit 3249" amount="25.000" tolerance=""/> <recipeline material="New Zealand or Grolleg kaolin" lookup="Grolleg Kaolin" amount="10.000" tolerance=""/> <recipeline material="45 micron Silica" lookup="Silica" amount="5.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2014-03-04, Modified: 2021-10-01 11:57:18

Cone 04 UltraClear Low-Expansion

Code #

G3879C

Materials Amt
Fusion Frit F-524 750.000 73.17%
Fusion Frit F-69 140.000 13.66%
New Zealand or Grolleg kaolin (Grolleg Kaolin) 90.000 8.78%
Silica 45 micron (Silica) 45.000 4.39%

Total:1,025.00

Auto Unity Formula

BaO 0.04
CaO 0.38
MgO 0.18
K2O 0.09
Na2O 0.10
(KNaO) 0.19
SrO 0.22
B2O3 0.74
Al2O3 0.47
SiO2 3.76

Ratios

Si:Al: 8.0:1
SiB:Al: 9.6:1
R2O:RO: 0.2:0.8

Expansion

6.2 (Molar:5.8)

LOI

1.1

Cost

2.73 per kg

Notes

*An adjusted version of G3879 to reduce thermal expansion. It switches the silica to 45 micron (325 mesh) for better dissolution in the melt. And it increases the frit low-expansion F-69 (the Fusion equivalent of Ferro 3249) at the expense of F-524.

This also switches to whiter burning kaolin (New Zealand), that improves transparency and removes the amber coloration.

It still fires to the same crystal clear. And is fitting pieces better, especially the L4410L body.

We mixed 1500g powder to 1125g water (57%/43%) to get a specific gravity of 1.54. This produced 1.7 litres.
After allowing to saturate overnight and screening to 80 mesh it worked well.

Comments

2021-09-06

Joe: August 12/21
Tried the same powder/water ratio as G3879B: 1500/1125 (57%/43%). SG 1.54. 1.7 litres.
Initial glaze was too thin, allowing to saturate overnight thickened it and a vigorous stir settled a 4-5 seconds. Screened 80 mesh,( a lot of material on screen for this amount of powder).
New Zealand Kaolin does not break down easily. Once screened 80 mesh, these agglomerations disappear.

Pictures

G3879C is firing much whiter, why?

Cone 04. All three clear glazes are on the same body. Left to right: Amaco LG10, G3879C recipe, Crysanthos SG213. The middle one employs Fusion Frit F-524, it is more expensive. But look at the benefit: It fires much more transparent so the piece is much whiter. And it is not crazing or pin holing. And it is glossier.

G3879C, G1916Q, G1916QL1 on L213 Cone 04

The move from ball clay to New Zealand kaolin (centre mug vs right mug) greatly improves the clarity (because of the reduction in iron and titanium).

G3879C vs. G1916QL on L4410K low fire white

The iron from the ball clay is the reason the glaze on the right is off-white.

Cone 04 G1916QL1 vs G3879C melt flow comparison

G1916QL1 is more melt fluid so it is likely useful before cone 04. But G3879C is also very fluid (it also has a higher surface tension). Both are producing textbook high quality flows.

G3879C, G1916QL1 on L4410L Bisque 1800 glaze cone 04

No crazing after weeks of use, ultragloss. Frit 524 has much less iron, that is why that mug is so much whiter.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Cone 04 UltraClear Low-Expansion" id="202474" key="SBM5AEBW" date="2021-10-01" codenum="G3879C"> <recipelines> <recipeline material="Fusion Frit F-524" amount="750.000" tolerance=""/> <recipeline material="Fusion Frit F-69" amount="140.000" tolerance=""/> <recipeline material="New Zealand or Grolleg kaolin" lookup="Grolleg Kaolin" amount="90.000" tolerance=""/> <recipeline material="Silica 45 micron" lookup="Silica" amount="45.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2019-04-15, Modified: 2021-10-01 11:57:07