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.
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.
Used materials and the chemical formula.
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.
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.
serie 1 - 4
1) The closest approximation of Gerstley borate with the frittes 3221/1451.
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).
This glaze hasn’t melted well, the blue colour is more dull.
The materials were chosen in such way that there isn’t any forming of CO2, as in the case of using whiting or dolomite.
The two new materials introduce more SiO2 and the glaze gives a poor crackle and melts bad.
Same materials as 3) with more colemanite to get a lower melting
The larger amount of colemanite gives this glaze a better melting behaviour.
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)
6) Increase of the B2O3 content with fritte 3221 as source. (B2O3=.79 mol)
7) Increase of the B2O3 content with fritte 3221 as source. (B2O3=.97 mol)
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).
8) Increase of the B2O3 content with Colemanite as source. (B2O3=.99 mol)
9) Increase of the B2O3 content with Colemanite as source. (B2O3=1.14 mol)
10) Increase of the B2O3 content with Colemanite as source. (B2O3=1.29 mol)
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%
12) Na2O=0 mol
13) Na2O=.13 mol
Decrease of the content of fritte 1451 to zero, but now fritte 1510 as a source of Na2O.
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.
14) content of talc in recipe 12) is now 0.
15) “same” recipe of 12) now with fritte 3221 as B2O3 source, colemanite=0
16) same as 15) now without talc.
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.
17) ratio 3221 :NS = 8 : 12 (thin/thick layer)
18) ratio 3221 :NS = 10 : 10 (thin/thick layer)
19) ratio 3221 :NS = 12 : 8 (thin/thick layer)
Conclusion: thin layers give a better crackle than thick layers.
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 the forming of crackles there is no big difference between the recipes.
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).
exp.23) same recepee as 22)
There wasn’t any difference in results, pity? No! because simplicity is also beautiful.
At first instance I tried to “remake” Gerstley borate with a combination of materials which gives the same chemical formula.
This can be realised by the addition of :
fritte 1451, dolomite, whiting, fritte 3221 (or colemanite).
The gas-bubbles formed during the firing of the glaze are produced by the decomposition-gasses CO2 (whiting/dolomite) or H2O (colemanite).
This can be prevented by using wollastonite and talc as a source of CaO and MgO (although then extra SiO2 is added).
Calcium borate fritte 3221 (Johnson/Matthey) is very suitable as a source of B2O3 and is much better than colemanite (pinholes).
In the experiments you can see that only nepheline syanite and fritte 3221 are very useful.
The spreadability of the glaze is improved by replacing a part of the nepheline syanite by ball clay.
The end recipe is surprisingly simple:
|Fritte 3221||100 gram|
|nepheline syanite||80 gram|
|Ball clay||20 gram|
Objects made with the end recipe