I played around with the chemistry of a pinholing glaze and noted the differences in the surface of the glaze, attempting to determine the cause of the pinholes.

1. The Woo Yellow
Pinhole Experiment

2. Original Recipe Source:
ceramicartsdaily.org/wp-content/uploads/2013/01/33TriedTrueGlazes.pdf
Original Recipe:
Original UMF:
Ceramic Arts Daily:
33 Tried and True
Glaze Recipes
(2nd Edition)
What? No boron in a cone 6 glaze?

3. Examples of Woo Yellow original recipe.
Pinholes when glaze is very thick.
No pinholes where thin.

4. Typical firing temp:
Hot Cone 6
Clay body:
Plainsman M340
Type of pinholes:
Mainly bubbles contained
within the glaze.
Most don’t tend to
penetrate through to the
clay body.

5. Woo 3

6. Woo 3 + 4% RIO
+ 10% Zircopax
Woo 3 + 4% RIO
+ 20% Zircopax
Woo 3 + 4% RIO
+ 20% Zircopax
(500g sample)
+ 1 drop of Darvan + 3 drops of Darvan

7. Woo 3 + 4% RIO + 20% Zircopax
+ 1 drop of Darvan + 3 drops of Darvan
Did Darvan fix the pinholing issue? It’s hard to say. It appears
that way, but it could be that the glaze coating is thinner in the 2nd
sample, resulting in less pinholes. Remember, it only pinholes
where thick. More testing required.

8. Woo 3
+ 4% RIO
+ 20% Zircopax
+ 1 drop of Darvan
(500g sample)
Cracking
Dunting
Shivering
OBSERVATION:

9. Woo 4

10. Woo 4 Base
Woo 4 + 4% RIO
Woo 4 + 4% RIO
+ 10% Zircopax
Woo 4 + 4% RIO + 10% Zircopax
+ 1 drop Darvan + 3 drops Darvan

11. Woo 4 + 4% RIO + 10% Zircopax
(500g sample)
+ 3 drops Darvan + 4 drops Darvan + 5 drops Darvan + 6 drops Darvan + 10 drops Darvan
10 drops of Darvan didn’t have any significant effect on pinholing

12. Woo 3 + 4% RIO
+ 20% Zircopax
+ Darvan
Fired in Reduction
Woo 4 + 4% RIO
+ 10% Zircopax
+ Darvan
Fired in Reduction
SAME GLAZES - NO PINHOLES
Possible Reasons: Hotter firing?
Longer time between adding of Darvan and dipping of tile – Darvan had more time to
incorporate into the mixture?

13. Woo 8b
Base Base
+ 10% Zircopax
Base
+ 10% Zircopax
+ 4% RIO
OBSERVATION: The pinholes don’t show up until we add RIO.
This glaze seems to be underfired.

14. Woo 9

15. Woo 9 base unfired Woo 9 base Woo 9 plus 4% RIO
Woo 9 + 4% RIO
+ 10% Zircopax
Woo 9 + 4% RIO
+ 15% Zircopax
Woo 9 + 4% RIO
+ 20% Zircopax

16. Woo 9 base Woo 9 plus 4% RIO
OBSERVATION: Dimpling occurs when RIO is added.
Appears to be healed over pinholes.

17. Woo 9 plus 4% RIO Woo 9 + 4% RIO
+ 10% Zircopax
OBSERVATION: The pinholes increase when we add zircopax.
Is the refractory nature of zircopax raising the melting temp of the glaze?
The bubbles healed over without the zircopax.
They didn’t heal over with the zircopax.
Could this be fixed by a longer/hotter firing?

18. CONCLUSION: RIO seems to be the cause of pinholes in this
glaze. The solution to eliminating these pinholes could be simply
a hotter or slower firing, giving bubbles more time to heal over. It
could possibly be solved by adding more Darvan and/or allowing
the Darvan to sit in the glaze longer before testing. More testing
is needed before any conclusion can be drawn.