Floating Blue - Substituting Gerstley Borate

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Gerstley Borate is running out soon (as of 2022). It is better to source B2O3 from frits than use a GB substitute. We can fix issues with the original floating blue recipe at the same time.


Project Name

Floating Blue - Substituting Gerstley Borate

Project Codenumber

UnAssigned

Notes

Floating Blue (also called Blue Hare's Fur) is probably the most well-known and popular cone 6 pottery glaze. It was popularized by the book The Potter's Complete Book of Clay and Glazes by James Chappell (in the current edition it is on page 210 under the heading "Floating Glazes"). The author calls it "spectacular and extraordinary". In fact, the vase that graces the cover of the book uses this glaze. Like most other popular glossy Gerstley Borate (GB) base glazes, this one also uses nepheline syenite, silica and kaolin. Its borate content is about middle-of-the-road compared to the range of recipes that we have studied.

This glaze was both famous and infamous! Because the base contains GB many people have gelling issues with the slurry. The high feldspar content (this has almost 50%, that is alot!) means high K2O and Na2O, these are the oxides that raise thermal expansion (and therefore crazing). It was a love-hate relationship for many people, the pull of the appearance is enough to make many endure all of its problems.

The term 'floating' could refer to two possibilities in our observation: The blue cobalt-colored surface appears to float on a translucent brown glass layer. This layer is visible where the color breaks to brownish hues on thinner sections at edges and irregularities in the surface. More likely, however, it refers to a white opalescent 'boron-blue' layer that often appears to float over the cobalt blue background (boron-blue calcium borate crystals form in borate glazes when calcium is present and the glaze is very fluid). This effect can best be amplified by melting a ball of glaze on a tile to get a very thick pool of glass. Cappell says "the colors seem to float on a surface of a darker background of great depth, reminiscent of a deep pool of water".

Floating Blue Original Cone 6 Glaze

Code #

G2826R

P Materials Amt
Gerstley Borate 27.000 26.81%
Nepheline Syenite 47.900 47.57%
EPK 5.500 5.46%
Silica 20.300 20.16%
P Additions
* Iron Oxide Red 2.000 1.99%
* Rutile 4.000 3.97%
* Cobalt Oxide 1.000 0.99%

Total:107.70

Auto Unity Formula + Analysis

CaO 0.41 5.54%
MgO 0.10 0.99%
K2O 0.10 2.31%
Na2O 0.38 5.74%
(KNaO) 0.49
TiO2 0.00 0.05%
P2O5 0.00 0.04%
B2O3 0.43 7.19%
Al2O3 0.55 13.39%
SiO2 3.84 55.50%
Fe2O3 0.01 0.20%

Ratios

Si:Al: 7.0:1
SiB:Al: 7.8:1
R2O:RO: 0.5:0.5

Expansion

7.6 (Molar:7.4)

LOI

9.1

Cost

0.00 per kg

Notes

*This recipe has been used for many decades. Its secret is the high melt fluidity (because Gerstley Borate is such an active melter) and the colorant addition. The rutile variegates the color imparted by the iron and cobalt.

Anyone who has used this glaze will testify to the fact that it is "fickle" as Chappell notes (this is an understatement, see above). He makes a number of recommendations on using this recipe: Use distilled or low mineral water, force all material through an 80 mesh screen, stir thoroughly before and during use to prevent settling out of the iron content, apply the thickness of a dime, fire to cone 6 oxidation exactly, and cool normally. One interesting comment Chappell makes is: "Don039;t substitute any other chemicals for those given". Since that is exactly what we plan to do I might also comment that we have had success at lower temperatures and slower cooling should enhance the effect.

People who use this glaze employ a variety of methods to increase the variation of surface color (i.e. stippling a second layer, brushing a wash of another coloring oxide, double dipping, applying a wash of rutile, etc.).

This recipe is coming to end-of-life since Gerstley Borate (GB) will soon be unavaiable. While substitutes are available it is better to source the B2O3 from frits (a classic example of the value of glaze chemistry).

This calculates to a very high thermal expansion, crazing is likely.
The high Gerstley Borate content means it is likely to gel.

Pictures

G2826R Floating Blue Original Recipe glaze sample

G2826R Floating Blue melt flow test

Compares Gerstley Borate and Boraq 2
This is a good recipe for comparing Gerstley Borates substitutes.

Alternate Code Number:G2587

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Floating Blue Original Cone 6 Glaze" id="67502" key="xokdSX72" date="2023-07-03" codenum="G2826R" picturebasename=""> <recipelines> <recipeline material="Gerstley Borate" amount="27.000" tolerance=""/> <recipeline material="Nepheline Syenite" amount="47.900" tolerance=""/> <recipeline material="EPK" amount="5.500" tolerance=""/> <recipeline material="Silica" amount="20.300" tolerance=""/> <recipeline material="Iron Oxide Red" amount="2.000" added="true"/> <recipeline material="Rutile" amount="4.000" added="true"/> <recipeline material="Cobalt Oxide" amount="1.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2000-12-21, Modified: 2023-07-03 12:54:57

Floating Blue Using Gillespie Borate

Code #

G2826R1

P Materials Amt
Gillespie Borate 27.000 26.81%
Nepheline Syenite 47.900 47.57%
EPK 5.500 5.46%
Silica 20.300 20.16%
P Additions
* Iron Oxide Red 2.000 1.99%
* Rutile 4.000 3.97%
* Cobalt Oxide 1.000 0.99%

Total:107.70

Auto Unity Formula

CaO 0.45
MgO 0.11
K2O 0.09
Na2O 0.35
(KNaO) 0.44
B2O3 0.36
Al2O3 0.51
SiO2 3.51

Ratios

Si:Al: 6.8:1
SiB:Al: 7.5:1
R2O:RO: 0.4:0.6

Expansion

7.6 (Molar:7.5)

LOI

9.4

Cost

0.00 per kg

Notes

*At 1.47 specific gravity this produces a far better slurry than does the same recipe using Gerstley Borate. There is no gelling and no settling into a hard layer.

However the difference in chemistry is significant. B2O3, Al2O3 and SiO2 are all lower. However, the melt fluidity is the same, likely because the percentage of Ulexite is higher (that melts better than Colemanite).

The thermal expansion is up, that could mean more tendency to craze. It would be easy to adjust other things in this recipe to match the chemistry, that could b done later.

Pictures

Gillespie Borate in the Original Floating Blue Recipe

Top left: Gerstley Borate (GB) (top) and Gillespie Borate bottom on a buff burning body.
Top right: Same but on a red burning body.
Centre: Melt fluidity GLFL test of the two glazes (GB) on the left.
Bottom: The two recipes and their calculated chemistries.
Clearly, the Floating Blue itself is firing greener than usual. And the Gillespie Borate version is much bluer. You may be used to something in between these two. If the green tones could likely be restored by a reduction in the cobalt and increase in the iron oxide. The good news that at 1.47 specific gravity, Gillespie Borate produces a far better slurry than does the same recipe using Gerstley Borate. There is no gelling and no settling into a hard layer.
The chemistry comparison at the bottom highlights some concerns, the difference is not insignificant. B2O3, Al2O3 and SiO2 are all lower (this could be part of the reason for the differences in color also). For better or worse, the melt fluidity is the same: Very high. This is likely because the percentage of Ulexite is higher (that melts better than Colemanite).

Alternate Code Number:G2587

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Floating Blue Using Gillespie Borate" id="242566" key="qLnaMhPx" date="2023-08-02" codenum="G2826R1" picturebasename=""> <recipelines> <recipeline material="Gillespie Borate" amount="27.000" tolerance=""/> <recipeline material="Nepheline Syenite" amount="47.900" tolerance=""/> <recipeline material="EPK" amount="5.500" tolerance=""/> <recipeline material="Silica" amount="20.300" tolerance=""/> <recipeline material="Iron Oxide Red" amount="2.000" added="true"/> <recipeline material="Rutile" amount="4.000" added="true"/> <recipeline material="Cobalt Oxide" amount="1.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2000-12-21, Modified: 2023-08-02 15:52:38

Floating Blue using Frit 3134

Code #

G2826U

P Materials Amt
Nepheline Syenite 25.860
Ferro Frit 3134 36.180
EPK 21.870
Silica 13.120
Talc 2.970
P Additions
* Iron Oxide Red 1.900
* Cobalt Oxide 0.900
* Rutile 3.700

Total:106.50

Auto Unity Formula

CaO 0.48
MgO 0.09
K2O 0.05
Na2O 0.37
(KNaO) 0.42
B2O3 0.44
Al2O3 0.55
SiO2 3.55

Ratios

Si:Al: 6.5:1
SiB:Al: 7.3:1
R2O:RO: 0.4:0.6

Expansion

7.4 (Molar:7.4)

LOI

3.6

Cost

0.00 per kg

Notes

*This the floating blue recipe derived in the lesson in the desktop INSIGHT manual many years ago. That lesson taught users to see glazes as formulas of oxides rather than recipes of materials. Although this recipe looks completely different than the original, its materials source a chemistry that is very similar.


Improvements seem possible.
-Use 325 mesh silica and increase it enough to match the SiO2 in the original.
-The colorants could be rounded off to 2, 4 and 1. And the other materials to a tenth.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Floating Blue using Frit 3134" id="67575" key="2WfuDFWS" date="2022-08-28" codenum="G2826U" picturebasename=""> <recipelines> <recipeline material="Nepheline Syenite" amount="25.860" tolerance=""/> <recipeline material="Ferro Frit 3134" amount="36.180" tolerance=""/> <recipeline material="EPK" amount="21.870" tolerance=""/> <recipeline material="Silica" amount="13.120" tolerance=""/> <recipeline material="Talc" amount="2.970" tolerance=""/> <recipeline material="Iron Oxide Red" amount="1.900" added="true"/> <recipeline material="Cobalt Oxide" amount="0.900" added="true"/> <recipeline material="Rutile" amount="3.700" added="true"/> </recipelines> </recipe> </recipes>

Born: 2001-03-02, Modified: 2022-08-28 22:00:11

Ravenscrag Floating Blue

Code #

G2917

P Materials Amt
Ravenscrag Slip 40.000 36.70%
Ravenscrag Slip 1000F Roast 39.000 35.78%
Ferro Frit 3134 20.000 18.35%
Ferro Frit 3124 10.000 9.17%
P Additions
* Iron Oxide Red 2.000 1.83%
* Cobalt Oxide 1.000 0.92%
* Rutile 4.000 3.67%

Total:116.00

Auto Unity Formula + Analysis

BaO 0.00 0.22%
CaO 0.68 12.42%
MgO 0.10 1.27%
K2O 0.08 2.31%
Na2O 0.14 2.81%
(KNaO) 0.22
TiO2 0.01 0.29%
B2O3 0.24 5.44%
Al2O3 0.32 10.70%
SiO2 3.06 59.62%
Fe2O3 0.01 0.62%

Ratios

Si:Al: 9.5:1
SiB:Al: 10.2:1
R2O:RO: 0.2:0.8

Expansion

7.1 (Molar:6.9)

LOI

4.3

Notes

*This is also known as GR6-M at PlainsmanClays.com.

It was made by simply adding the three colorants to the GR6-A Ravenscrag base recipe. Later, 10% frit 3124 was introduced to increase melt fluidity (a secret of the original to the floating effect). Curiously, the B2O3 and KNaO contents are much lower, yet this still produces the same result!

This produces a much better slurry than the Gerstley Borate based traditional floating blue. This works well on M340, even better on M390. It is more transparent and has less floating effect on porcelain.

If grit forms on rims of mugs, you may need to ball mill the glaze. Or sieve it at 80 mesh.

We mix this as a slurry using the same procedure as for GR6-A base recipe.

Batch Ticket Notes

For brushing version:
340 powder
440 water
5 CMC
5 Veegum
Brushing version Oct 2022 used Frit f-12 instead of 3134.

Pictures

Ravenscrag vs. Alberta Slip floating blues at cone 6

Usable, reliable, non-crazing floating blue glazes are difficult to achieve at cone 6. Not these, they pass all the tests yet fire like the original classic G2826R floating blue from David Shaner. Both have been applied at moderate thickness on Plainsman M325 (using a slurry of about 1.43-1.45 specific gravity, higher values end up getting them on too thick). The Ravenscrag version (left) highlights contours better (the edges are black because of the black engobe underneath). It also produces the blue color whether or not the kiln is slow-cooled (although drop-and-hold PLC6DS schedule usually fires more blue). The Alberta Slip version has zero cobalt so it is less expensive to make (but it does require the C6DHSC slow-cool firing schedule). It produces a deeper color over the L3954F black engobe on these pieces. Both of these produce a wide range of effects with different thicknesses, bodies and firing schedules.

Ravenscrag Floating Blue on M370 Cone 6

G2917 RavenBlue cooled slowly, quickly

Rim grit on Ravenscrag glaze not sieved to 80 mesh

Ravenscrag is processed at 42 mesh so particles can disrupt a glaze surface if it is applied or runs very thin.

G2917 Raven blue at cone 7 on P6953 M325

GR6-M cooled quickly, slowly

The left mug was fired using our drop-and-soak PLC6DS, the other with the slow-cool C6DHSC schedule.

Ravenscrag Floating blue as brushing glaze

This can be made into a pint (500ml) of brushing glaze with 340g powder, 5g CMC gum and 5g of Veegum. Blender mixing is a must to get the gelled consistency.

G2917 floating blue vs. Amaco PC-20

The PC-20 Rutile Blue is on the left. It has higher melt fluidity and more white crystallization. G2917 could be altered by substituting some or all of the rutile for titanium dioxide.

Alternate Code Number:GR6-M

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Ravenscrag Floating Blue" id="60534" key="iPma7gna" date="2023-07-20" codenum="G2917" picturebasename=""> <recipelines> <recipeline material="Ravenscrag Slip" amount="40.000" tolerance=""/> <recipeline material="Ravenscrag Slip 1000F Roast" amount="39.000" tolerance=""/> <recipeline material="Ferro Frit 3134" amount="20.000" tolerance=""/> <recipeline material="Ferro Frit 3124" amount="10.000" tolerance=""/> <recipeline material="Iron Oxide Red" amount="2.000" added="true"/> <recipeline material="Cobalt Oxide" amount="1.000" added="true"/> <recipeline material="Rutile" amount="4.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2013-08-30, Modified: 2023-07-20 09:37:17

Titanium Dioxide in GA6-C

Code #

L4655

P Materials Amt
Alberta Slip 40.000
Alberta Slip 1000F Roasted 40.000
Ferro Frit 3134 20.000
P Additions
* Titanium Dioxide 4.000

Total:104.00

Auto Unity Formula

CaO 0.48
MgO 0.24
K2O 0.09
Na2O 0.19
(KNaO) 0.28
B2O3 0.20
Al2O3 0.39
SiO2 2.70
Fe2O3 0.07

Ratios

Si:Al: 7.0:1
SiB:Al: 7.5:1
R2O:RO: 0.3:0.7

Expansion

7.6 (Molar:7.2)

LOI

4.9

Notes

*The original motivation for this test was to compare three different bags of titanium dioxide. However it produced an unexpectedly good floating blue.

We used the GA6C Rutile Blue recipe and substituted the Ceramic Rutile portion for an equal amount of Titanium Dioxide, (4%).

We used both the C6DS and C6DSSC firing schedules. The glaze was tried on M340, M390, and Coffee clay tiles, on these bodies there is no issues with crazing. But on porcelains there likely would be.

There is some doubt about the correctness of this recipe, it might be missing the 2% iron. We are checking.

Pictures

Titanium instead of rutile in GA6-C

These mugs are M390 clay. The top left one is the standard GA6-C (with rutile) fired using the C6DHSC firing schedule (the bottom left normal cool produces little color). But the ones on the right switch the 4% rutile for titanium dioxide (the L4655 recipe). The top right was fired using the C6DHSC slow cool schedule, the bottom right was the PLC6DS schedule.

Three bags of titanium dioxide compared

Left tiles are GA6-C cone 6 C6DH firing.
Right tiles are cone 6 are C6DHSC firing.

Would titanium work instead of rutile? Yes.

Rutile blue glazes are actually titanium blues (because rutile sources TiO2). Thus titanium dioxide can be used instead of rutile (provided the needed iron oxide, also in rutile, is being supplied by another material).

The center top tile is the standard GA6-C (with rutile) fired using the required C6DHSC firing schedule. But the one to its left and right contain 4% titanium dioxide. The one on the left was fired using the C6DHSC slow cool schedule, the one on the right the PLC6DS schedule (the bottom two reflect the same, but on a buff burning body.

A lower percentage of titanium could likely be used.

L4655 glazed mugs

These mugs were in the same firing. On the porcelain (left) the glaze fires the expected floating blue. The degree of difference on the right has two contributing apparent factors. While other clay bodies of similar color do not affect this glaze as much, the body used in the mug on the right contains Plainsman 3B, at cone 6 it vitrifies (releasing iron compounds) and it releases iron in soluble salts that are interacting with the glaze. Titanium is very sensitive to the presence of iron and this body is making it available in an effective form.

L4705 and L4705A (front)

Slow cool firing
Mug is L4655.
Clearly, Frit 3134 is required.

Untitled

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Titanium Dioxide in GA6-C" id="239613" key="3zdzceyk" date="2024-03-08" codenum="L4655"> <recipelines> <recipeline material="Alberta Slip" amount="40.000" tolerance=""/> <recipeline material="Alberta Slip 1000F Roasted" amount="40.000" tolerance=""/> <recipeline material="Ferro Frit 3134" amount="20.000" tolerance=""/> <recipeline material="Titanium Dioxide" amount="4.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2023-05-25, Modified: 2024-03-08 21:18:10