Fixing the Fix: A better 50:30:20 cone 6 glaze recipe

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The popular Gerstley Borate 50:30:20 glaze recipe will soon be impossible. But it can be made using Gillespie Borate and frits. And improved.


50:30:20 Gerstley Borate Cone 6 base

Code #

G2826A

Materials Amt
Gerstley Borate 50.000
EPK 20.000
Silica 30.000

Total:100.00

Auto Unity Formula

CaO 0.69
MgO 0.17
K2O 0.01
Na2O 0.13
(KNaO) 0.14
B2O3 0.76
Al2O3 0.31
SiO2 3.06

Ratios

Si:Al: 9.9:1
SiB:Al: 12.4:1
R2O:RO: 0.1:0.9

Expansion

6.0 (Molar:6.1)

LOI

17.7

Cost

0.00 per kg

Notes

*This is the base for many recipes (e.g. Butterscotch (Rust) added: 4% rutile, 4% tin oxide and 1% lithium carboante.

This fires with plenty of boron blue and bubble clouding, but that was seen as part of the aesthetic of the surface. It has a low thermal expansion and fitted most clay bodies.

Mixing this 50:50 with water produces 1.44 specific gravity, but it will be pure gel. Adding 0.5% Darvan helps to thin it for long enough to dip bisque samples (but the glaze goes on very thin). The slurry re-gels within seconds.

For making flow tester balls this slurry dewaters very slowly on a plaster bat.

GB is a clay, capable of suspending and dry hardening a glaze at only 25% concentration. Yet this has double that plus adds kaolin which would further increase drying shrinkage! It is amazing that it was even possible to make a useable slurry out of this!

Pictures

G2826A vs G2826A1 low test

A1 supplies boron using frit 3134.
Since the frit contains less boron the full B2O3 cannot be supplied and it has a little more Al2O3 and SiO2.
But the A1 has 6 times lower LOI.
G2826A is a good recipe for comparing GB substitutes.

G2826A Gerstley Borate 50:30:20 base as jelly

G2826A on M390 - lots of boron blue

You might think this looks nice. But it comes at a cost (bubbling, jelling, running, cracking on drying, crawling). Boron blue is considered a glaze fault.

That being said, many people like gerstley borate for this very reason.

Fired at 1700F G2826A 50:30:20 GB

Fired at 1700F
G2826A 50:30:20 GB
G2826A1 same chem using frit
G2826A2 50:30:20 using Gill Borate (should be crawling??)
G2826A3 50:30:20 makeover using GillBor
G2826A3-2 same but with calcine kaolin

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="50:30:20 Gerstley Borate Cone 6 base" id="67237" key="Lc2hARxa" date="2023-09-25" codenum="G2826A" picturebasename=""> <recipelines> <recipeline material="Gerstley Borate" amount="50.000" tolerance=""/> <recipeline material="EPK" amount="20.000" tolerance=""/> <recipeline material="Silica" amount="30.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2000-04-18, Modified: 2023-09-25 16:59:00

50:30:20 Frit 3134 cone 6 base

Code #

77C04E

Materials Amt
Ferro FRIT 3134 50.000 47.62%
EPK 30.000 28.57%
Silica 25.000 23.81%

Total:105.00

Auto Unity Formula

CaO 0.68
Na2O 0.32
(KNaO) 0.32
B2O3 0.63
Al2O3 0.46
SiO2 3.96

Ratios

Si:Al: 8.5:1
SiB:Al: 9.9:1
R2O:RO: 0.3:0.7

Expansion

6.7 (Molar:6.8)

LOI

4.2

Cost

0.00 per kg

Notes

*This was a product of substituting Ferro Frit 3134 for Gerstley Borate in the long time G2826A base transparent recipe.

The result is a poor match of the overall chemistry. The KNaO is doubled, the Al2O3 is 50% higher, the SiO2 25% higher and there is no MgO.

A big downside of this recipe is cost, it is 50% frit. Also this recipe has a higher thermal expansion, which means it will craze on some bodies that the original G2826A fitted well.

This being said, this recipe has been popular for 50 years or more. It is the base of many, one common one is a bright opal blue made by adding 2-3% cobalt oxide.

An upside of this version compared to the original 50:30:20 G2926A recipe is the 20% kaolin, that produces a nice slurry that suspends and applies well.

Pictures

Typecodes

RCP-Undefined

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="50:30:20 Frit 3134 cone 6 base" id="63485" key="sZEQzb93" date="2023-09-08" typecodes="RCP" codenum="77C04E" picturebasename=""> <recipelines> <recipeline material="Ferro FRIT 3134" amount="50.000" tolerance=""/> <recipeline material="EPK" amount="30.000" tolerance=""/> <recipeline material="Silica" amount="25.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 1980-01-01, Modified: 2023-09-08 03:24:35

50:30:20 Frit 3134 base (fixed)

Code #

G2826A1

Materials Amt
Ferro Frit 3134 29.000
EPK 19.000
Fusion Frit F-69 20.000
325 mesh silica (Silica) 22.000
Wollastonite 10.000

Total:100.00

Auto Unity Formula

CaO 0.63
MgO 0.21
Na2O 0.16
(KNaO) 0.16
B2O3 0.61
Al2O3 0.33
SiO2 3.22

Ratios

Si:Al: 9.7:1
SiB:Al: 11.5:1
R2O:RO: 0.2:0.8

Expansion

6.0 (Molar:6.1)

LOI

2.8

Cost

0.00 per kg

Notes

*The 77C04E base (50:30:20 Frit 3134:silica:kaolin) has multiple issues as an alternative to the original 50:30:20 Gerstley Borate G2826A recipe. This addresses those.

The boron here is less that G2826A, this is as much as common frits will supply - however, this amount produces fewer bubbles, better durability, much less boron blue. The MgO is a little higher, this helps keep the thermal expansion down.

100:80 powder:water mix produces a specific gravity of 1.53, a creamy slurry that applies perfectly. But it would likely be better with more water and Epsom salts (to create a thixotropic slurry).

Of course, this produces a more sterile looking glaze, without the variegation and boron blue. But variegation can be restored with the addition of 1-4% titanium dioxide, it produces a cloudiness that looks better than boron blue. Colorants can then be added to that. Rutile, an impure form of titanium, could also be used, it can both stain and variegate the fired glass.

Pictures

G2826A vs A1 50:30:20 base flow test

The flow of the fritted version (on the right) has no boron-blue, more surface tension, less LOI disruption. Fluidity is only a little less even though this has less boron.

G2826A, G2826A1 on M340, M370

Boron blue gone on the fritted version.
Of course, if you want this effect it can be achieved by increasing the amount of B2O3 in the formula.

G2826A vs G2826A1 demos Gerstley Borate vs Frits to source boron

If a better quality functional glaze is needed then the G2826A1 fritted version on the right is much better. That being said, many people seek the visual character of the one on the left. Perhaps a mix of the two could reduce some of the working and technical issues with the G2826A version.

G2826A vs G2826A1 low test

A1 supplies boron using frit 3134.
Since the frit contains less boron the full B2O3 cannot be supplied and it has a little more Al2O3 and SiO2.
But the A1 has 6 times lower LOI.
G2826A is a good recipe for comparing GB substitutes.

G2826A3 vs G2826A1 - Fired to 1700F

The G2826A1 employs a frit, the G2826A2 (left) Gillespie Borate.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="50:30:20 Frit 3134 base (fixed)" id="223705" key="rpbjAM2x" date="2023-09-08" codenum="G2826A1" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3134" amount="29.000" tolerance=""/> <recipeline material="EPK" amount="19.000" tolerance=""/> <recipeline material="Fusion Frit F-69" amount="20.000" tolerance=""/> <recipeline material="325 mesh silica" lookup="Silica" amount="22.000" tolerance=""/> <recipeline material="Wollastonite" amount="10.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2000-04-18, Modified: 2023-09-08 03:23:36

50:30:20 Gillespie Borate Cone 6 base

Code #

G2826A2

Materials Amt
Gillespie Borate 50.000
EPK 20.000
Silica 30.000

Total:100.00

Auto Unity Formula

CaO 0.71
MgO 0.17
Na2O 0.11
(KNaO) 0.11
B2O3 0.61
Al2O3 0.28
SiO2 2.60

Ratios

Si:Al: 9.2:1
SiB:Al: 11.3:1
R2O:RO: 0.1:0.9

Expansion

6.2 (Molar:6.3)

LOI

18.4

Cost

0.00 per kg

Notes

*It substitutes Gillespie Borate directly for Gerstley Borate in the popular G2826A 50:30:20 recipe.

When applied thinly (which is needed or it will run during firing) this glaze produces a crystal clear (very little boron blue compared to G2826A3). Of course, for functional ware. that means the body must have a smooth surface (or surface particles will be felt through the glaze).

For us, a test mix of 330g dry requires 400g of water for a thixotropic dipping glaze (which works really well). On thick applications it will crack because of the drying shrinkage however that is not a problem because it needs to be applied thinly anyway (for the above-mentioned reason).

For making a GBMF test this dewaters on a plaster bat in a few minutes (vs. the Gerstley version which takes an hour).

Even though Gillespie Borate is really plastic, to make a brushing glaze this still needs Veegum to enable increasing water content (otherwise it will go on too thick). Strangely it may need even more than the typical 1.5% CMC gum to dry slowly enough to get even application.

Pictures

G2826A, G2826A1 50:30:20 glaze

On the left is uses Gerstley Borate, on the right Gillespie Borate. This was fired to cone 6 using the PLC6DS schedule.
The GBMF test tiles (lower left and right) reveal how much off-gassing is still happening when melting starts. The GLFL test (centre) shows the melt flow of the two glazes.

G2826A, A2, A1 50:30:20 Gillespie Borate glaze

Strangely, the frited one on the right has the most bubbles. It is also the most fluid.

G2826A2 vs G2826A3 melt flow test

G2826A2 vs G2826R2

Left: G2826A2, the 50:30:20 GB clear recipe. This is a nice transparent, and it has the same boron blue as Gerstley Borate had.
Right: G2826R2 - Floating blue recipe but using Titanium and iron instead of the traditional rutile, iron and cobalt. It is not working.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="50:30:20 Gillespie Borate Cone 6 base" id="242662" key="oWfKPsgM" date="2024-03-08" codenum="G2826A2" picturebasename=""> <recipelines> <recipeline material="Gillespie Borate" amount="50.000" tolerance=""/> <recipeline material="EPK" amount="20.000" tolerance=""/> <recipeline material="Silica" amount="30.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2000-04-18, Modified: 2024-03-08 17:02:25

50:30:20 GB Makeover Pottery Glaze

Code #

G2826A3

Materials Amt
Gillespie Borate 37.000
EPK 15.000
325 mesh Silica (Silica) 35.000
Nepheline Syenite 10.000
Talc 3.000

Total:100.00

Auto Unity Formula

CaO 0.59
MgO 0.23
K2O 0.02
Na2O 0.15
(KNaO) 0.17
B2O3 0.50
Al2O3 0.32
SiO2 3.48

Ratios

Si:Al: 10.7:1
SiB:Al: 12.3:1
R2O:RO: 0.2:0.8

Expansion

6.0 (Molar:6.0)

LOI

13.9

Cost

0.00 per kg

Notes

*Melt fluidity is a major problem with the 50:30:20 recipe, it is capable of dissolving its way through an insulating firebrick (see below)! This adjustment attempts to add some sanity to this classic recipe (less melt fluidity) and employ Gillespie Borate as a substitute for Gerstley Borate.

It seemed that it would be possible to achieve the following benefits with a change in chemistry and recipe:
- More SiO2 to reduce the thermal expansion to equal that of the original 50:30:20 recipe.
- More Al2O3 to increase durability (always a problem with fluid melts).
- Less B2O3 flux to reduce melt fluidity.
-A high Si:Al ratio to increase gloss (to be sure to preserve it through the other changes).
-325 mesh silica dissolves more completely in the melt to make a more durable glass.

A benefit of the above changes is a lower LOI.

A problem for some and a benefit for others is the boron blue clouding this produces, it is much worse than the much more fluid melt G2826A2. But it also produces stunning variegation on darker burning bodies.

85:100 water:powder worked well to start.

Version 2.0 uses calcined kaolin instead. At about 50:50 water:powder it produces a beautiful slurry that applies evenly without dripping. However the crawling is about the same.

Pictures

G2826A3 50:30:20 Gillespie Borate test tile

On L3905H
Fires transparent with lots of boron blue.

Untitled

G2826A3 vs G2926Bgi2 glazed mugs

G2826A3 inside left.

A2 vs A3 - Slow cool

G2826A3 50:30:20 Gillespie Borate on test tiles

On M370, MNS L4449J black and M390.

G2826A2 vs G2826A3 melt flow test

G2826A3 50:30:20 glaze - EPK vs Calcined Kaolin

Fired at 1700F
Calcined kaolin version 2.0 (right) is applied a little thinner.
It does not seem to have helped with the crawling.

G2826A3 vs G2826A1 - Fired to 1700F

The G2826A1 employs a frit, the G2826A2 (left) Gillespie Borate.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="50:30:20 GB Makeover Pottery Glaze" id="243239" key="ZjvHcY99" date="2023-09-25" codenum="G2826A3" picturebasename=""> <recipelines> <recipeline material="Gillespie Borate" amount="37.000" tolerance=""/> <recipeline material="EPK" amount="15.000" tolerance=""/> <recipeline material="325 mesh Silica" lookup="Silica" amount="35.000" tolerance=""/> <recipeline material="Nepheline Syenite" amount="10.000" tolerance=""/> <recipeline material="Talc" amount="3.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2000-04-18, Modified: 2023-09-25 16:49:09