Gerstley borate

Goal: The substitution of Gerstley borate.

When we try to “remake” Gerstley borate from materials which are commercially readily available on the market, it is necessary to have the chemical formula of Gerstley borate.
A publication of Digitalfire gives a number of different Gerstley borate compositions. In my experiments to remake Gerstley borate I calculated the average of these values to obtain a single chemical formula. (this is called GB RR)
The chemical analysis is given in weight % and at first this is recalculated to the Seger formula.(the lost of water and the forming of CO2 gas- LOI=lost of ignition- is taken into account in the moleculair weight.)

As a starting point the calculations and the experiments were used to “remake” Gerstley borate, but the real purpose is to make an useful recipe for the Raku potter.
The experiments are reported in sequence of execution and although this is perhaps not optimal from a didactic point of view, the final recipes are fortunately very simple.
In a Raku glaze Gerstley borate is never used by itself but always in a combination with other materials like nepheline syanite in a weight-ratio around 1:1
The calculations have been carried out with the program alchemistglaze, a “homemade” excel 2000 program. (The program is available for free after sending a request form click here )

Used materials and the chemical formula.

Work method:

Choose materials with a simple chemical formulation and then calculate after that the amount of these materials to obtain ( as close as possible) the wanted recipe.
We use the Seger formulation, because this is the most simple way. To illustrate this procedure, we took recipe 2) as an example.

What we want to attain:	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 GB RR .12 .16 .72 .59 .33
First we choose for a Na2O source: let's take a fritte 1451 in the amount of .12 mol. When we want to know how much we have to weigh, than we multiply .12 mol with the molweight of this material =.12*381 (See list of materials)=46 gram	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 Fritte 1451 381 46 .12 .12 .24 .36
As a MgO source we have choosen Dolomite in the amount of .16 mol, a complication is that at the same time CaO is added (but we stay under the total amount of the CaO)	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 Dolomite 184 29 .16 .16 .16
We will now balance the amount of B2O3.The total amount is .59 mol but with fritte 1451 we have already supplied an amount of .24 and so there is a rest of .35 mol. We choose to use Colemanite and we need .35/3=.12 mol(see materials: 1 mol of Colemanite gives 3 mol of B2O3)	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 Colemanite 411 49 .12 .24 .36
Now there only remains CaO and we make this okay by adding .32 mol of whiting	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 Withing 100 32 .32 .32
The ultimate result is the sum and this is in good accordance with the desired recipe.	material... molweight weight Mol% Na2O MgO CaO B2O3 SiO2 Total .12 .16 .72 .60 .36

This isn´t very difficult, but it isn´t always possible to get a solution (especially when the materials are more complex).
Try to make the sum of the earth alkali oxides (Na2O K2O MgO CaO) around 1 .
In the course of the experiments the target to exactly "remake" Gerstley borate has been abandoned and instead we have focussed on the appearance of the glaze. The Seger formulation has been calculated each time, but it is only purpose to guide us in making logic changes to the recipes.

Experiments

serie 1 - 4

1) The closest approximation of Gerstley borate with the frittes 3221/1451.

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Withing	100	21	.21			.21
Dolomite	184	29	.16		.16	.16
Fritte 3221	125	44	.35			.35	.35
Total				.12	.16	.72	.59	.36
GB RR				.12	.16	.72	.59	.33

This glaze gives gas-bubbles, clearly visible in the picture with the blue colour. Crackles are not developed very well.

2) The closest approximation of Gerstley borate with the fritte 1451 and colemanite, the other materials are the same as 1).

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Withing	100	32	.32			.32
Dolomite	184	29	.16		.16	.16
Colemanite	411	49	.12			.24	.36
Total				.12	.16	.72	.60	.36
GB RR				.12	.16	.72	.59	.33

This glaze hasn’t melted well, the blue colour is more dull.

3) The materials were chosen in such way that there isn’t any forming of CO2, as in the case of using whiting or dolomite.
Colomanite is chosen as B2O3-source.

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talc	379	19	.05		.15			.20
Wollastonite	116	55	.47			.47		.47
Colemanite	411	49	.12			.24	.36
Total				.12	.15	.71	.60	1.03
GB RR				.12	.16	.72	.59	.33

The two new materials introduce more SiO2 and the glaze gives a poor crackle and melts bad.

4) Same materials as 3) with more colemanite to get a lower melting temperature.
(As B2O3 source colomanite is chosen. )

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talc	379	19	.05		.15			.20
Wollastonite	116	37	.33			.33		.33
Colemanite	411	82	.20			.40	.60
Total				.12	.15	.73	.84	.89
GB RR				.12	.16	.72	.59	.33

The larger amount of colemanite gives this glaze a better melting behaviour.
In the series this recipe gives the best results.

serie 5 - 10

In the series 5-7 the content of fritte 3221 has increased and because this is a Ca borate we have to lower the content of wallastonite at the same time . (The content of Na2O, MgO, CaO is always the same). The content of SiO2 decreases because the Wollastonite content decreases

5) Increase of the B2O3 content with fritte 3221 as source. (B2O3=.64 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talc	379	19	.05		.15			.20
Wollastonite	116	35	.30			.30		.30
Fritte 3221	125	50	.40			.40	.40
Totaal				.12	.15	.70	.64	.86
GB RR				.12	.16	.72	.59	.33

6) Increase of the B2O3 content with fritte 3221 as source. (B2O3=.79 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talc	379	19	.05		.15			.20
Wollastonite	116	18	.15			.15		.15
Fritte 3221	125	68	.55			.55	.55
Totaal				.12	.15	.70	.79	.71
GB RR				.12	.16	.72	.59	.33

7) Increase of the B2O3 content with fritte 3221 as source. (B2O3=.97 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talc	379	19	.05		.15			.20
Wollastonite	116
Fritte 3221	125	91	.73			.73	.73
Totaal				.12	.15	.73	.97	.56
GB RR				.12	.16	.72	.59	.33

Number 7 in the series gives the best result.Also here we can see that a higher content of B2O3 gives beter results.

In this serie the B2O3 content has increased by Colemanite (and not by the fritte 3221).
The content of Na2O,MgO,CaO is always the same

8) Increase of the B2O3 content with Colemanite as source. (B2O3=.99 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talk	379	19	.05		.15			.20
Wollastoniet	116	23	.20			.20		.20
Colemaniet	411	102	.25			.50	.75
Totaal				.12	.15	.70	.99	.76
GB RR				.12	.16	.72	.59	.33

9) Increase of the B2O3 content with Colemanite as source. (B2O3=1.14 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talk	379	19	.05		.15			.20
Wollastoniet	116	12	.10			.10		.10
Colemaniet	411	123	.30			.60	.90
Totaal				.12	.15	.70	1.14	.66
GB RR				.12	.16	.72	.59	.33

10) Increase of the B2O3 content with Colemanite as source. (B2O3=1.29 mol)

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	46	.12	.12			.24	.36
Talk	379	19	.05		.15			.20
Wollastoniet	116
Colemaniet	411	144	.35			.70	1.05
Totaal				.12	.15	.70	1.29	.56
GB RR				.12	.16	.72	.59	.33

In the series 5-7 the content of colemanite has been increased, at the same time the content of wallastonite has lowered to zero. (The content of Na2O, MgO, CaO is always the same).
By increasing the content of colemanite the amount of gas-bubbles is also increasing as you can see by the pinholes in the glaze.
Number 7 in the series gives the best result.

serie 11 - 13

Study of the influence of fritte 1451 (and thus of the Na2O content)

11) NaO=.06 mol%

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381	23	.06	.06			.12	.18
Talc	379	19	.05		.15			.20
Colemanite	411	164	.40			.80	1.20
Total				.06	.15	.80	1.32	.38
GB RR				.12	.16	.72	.59	.33

12) Na2O=0 mol

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1451	381
Talc	379	19	.05		.15			.20
Colemanite	411	172	.42			.84	1.26
Total					.15	.84	1.26	.20
GB RR				.12	.16	.72	.59	.33

13) Na2O=.13 mol

Decrease of the content of fritte 1451 to zero, but now fritte 1510 as a source of Na2O.
( .13 mol Na2O ,the same as in exp.10)
This experiment is carried out to see if the pinholes -by using colemanite - were caused by the low melting temperature of fritte 1451.
(The low meltingpoint of one of the components can have an effect on the evaporation of water normaly present in colemanite - 2CaO.3B2O3.2H2O -)
(The result of the experiment 12) wasn’t known, otherwise this result would make clear that the melting behaviour of fritte 1451 didn’t have any influence).

material	molweight	weight	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Fritte 1510	198	40	.2	.13		.02	.04		.42
Talk	379	19	.05		.15				.20
Colemaniet	411	144	.35			.70		1.05
Totaal				.13	.15	.72	.04	1.05	.62
GB RR				.12	.16	.72		.59	.33

The glazes are rather bad with in general many pinholes , there are a lot of gas-bubbles during the firing.

serie 14 - 16

In experiment 12) the Na2O content has been lowered to zero and this has no influence on the results, as a consequence also the amount of MgO has been decreased to see what happens.
This is done with Colemanite as well with fritte 3221 as a B2O3 source.

14) content of talc in recipe 12) is now 0.

materiaal	molgew	gewicht	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Talk	379
Colemaniet	411	206	.50			1.0	1.50
Fritte 3221	125
Totaal						1.0	1.5
GB RR				.12	.16	.72	.59	.33

15) “same” recipe of 12) now with fritte 3221 as B2O3 source, colemanite=0

materiaal	molgew	gewicht	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Talk	379	19	.05		.15			.20
Colemaniet	411
Fritte 3221	125	106	.85			.85	.85
Totaal					.15	.85	.85	.20
GB RR				.12	.16	.72	.59	.33

16) same as 15) now without talc.

materiaal	molgew	gewicht	Mol%	Na2O	MgO	CaO	B2O3	SiO2
Talk	379
Colemaniet	411
Fritte 3221	125	125	1.0			1.0	1.0
Totaal						1.0	1.0
GB RR				.12	.16	.72	.59	.33

The result of the experiments 15-16 are the best, there isn’t any gasformation during the firing and that’s the reason that there aren’t any pinholes in the glaze.
Experiment 14, with only colemanite, gives a lot of gas and pinholes and a rather bad crackle.

The use of colemanite in Raku glazes isn’t a good choice! (although the price of the material is low.) A special addition of Na2O, MgO, CaO isn’t necessary when we use nepheline syanite !!

serie 17 - 19

Now that we know that only with fritte 3221 and nepheline syanite it is possible to make a good glaze we want to examine if the ratio between the two materials (till now always in weight-ratio 1:1) has a pronounced effect on the results.
Also the thickness of the layer is a part of the experiment

17) ratio 3221 :NS = 8 : 12 (thin/thick layer)
Thin layer gives better crackles.

18) ratio 3221 :NS = 10 : 10 (thin/thick layer)
Thin layer gives better crackles.

19) ratio 3221 :NS = 12 : 8 (thin/thick layer)
Thin layer gives better crackles.

Conclusion: thin layers give a better crackle than thick layers.
There is a small difference in the ratio of the fritte 3221 and nepheline syanite, experiment 17/18 is a little better than 19 and fortunately it is cheaper too!!

serie 20 - 22

The recipe found up to now is good and simple, however it’s difficult to spread and so the possibility to apply the glaze with a brush isn’t optimum.
In this series ball clay is added to improve the spreadability.

material	exp. 20	exp. 21	exp. 22
fritte 3221	10	10	10
nepheline syanite	10	9	8
ball clay		1	2

In the forming of crackles there is no big difference between the recipes.
In experiment 22 the spreadability (and also the strength of the layer) is comparable with Gerstley borate (which is excellent to apply with a brush).

Serie 23 - 25

In an attempt to make the recipe more complicated (you are an alchemist or not.) I have played with some additions of Na2O in the form of salt (NaCl) or soda (NaHCO3.H2O).
From top to bottom:

exp.23) same recepee as 22)
exp.24) 1 gram addition of salt (naCl)
exp 25) 1 gram addition of soda (NaHCO3.H2O)

There wasn’t any difference in results, pity? No! because simplicity is also beautiful.