Pinholes in Newly Glazed Vases

[TonyC]

Took some pots out of the kiln and noted pinholes in both the celadon (Amaco Sky) and satin matte (Amaco white) vases.   This is one of my earliest glaze firings and I am curious as to the factors to investigate to better control this issue.   Here is some data:

1.  Both clay bodies are Sheffield 20231G clay

2.  Bisque fired in my E23S L&L kiln (programmable) to Cone 06 (verified with actual cones as accurate).   

3.  These 2 examples were more than a year old before the glazes were applied.   Wiped pots but NOT with damp sponge.  Possible problem?

4.  Applied 3 coats of brushed on Cone 5/6 glazes from Amaco; Sky Celadon and White Satin Matte (examples).  Amaco Shino glazes also, but NO pinholes in any of those 4 pieces.   I think each coat dried before next coat applied, but not sure how dry is 'enough'  (possible problem).   I never measured thickness of final glazed product so can't confirm if too thick.  Possible problem?

5.  These 2 examples were both then fired with other pieces on the top shelf (only 2 shelves) to the pre-programmed Slow Glaze Cone 5.  The temperature reached was a little higher than the program 2165F (Actual 2171).  Total time 8 hr 44 min (included 15 min pre-heat, NO hold time or custom cool down).   Witness cones on both shelves show that witness cone 6 had begun to bend.   (I will try to attach a pic).   

6.  Pinholes seemed to be biggest issue although I also saw some cracking with the celadon glaze.   I will try to attach pix.   

7 (new).  Took pix from top view to see thickness.  Also, both of the vases were inner glazed with Amaco Clear glaze.

Based on this info (or other you might request), can someone please offer some possible root causes and ways to correct them?  I am happy to provide any info that can help you help me.   Thank you to any and all comments/advice.

 

[Min]

Pinholes look worse at the bottom of the vase, I'm guessing this is an area you trimmed? Divots in the clay from trimmed areas in grogged clay can lead to pinholes. Run a rib over any trimmed areas to smooth the surface of the clay after trimming. The white glaze looks really stiff, could be it just needs to go a bit hotter. Really look over the glaze before firing, if you see any pinholes in the raw glaze then rub them over with your finger and get rid of them.

Crazing is from the glaze not fitting the claybody. Glaze has too high an expansion for the clay it's on. Since this is a commercial glaze it's difficult at best to fix it. Is this the clay you used? If so, it's rated to go to ^10 so I would check your vases for leaking. Just put some water in them and leave them sit on some newsprint for a few days. Cone 6 (midrange) clay for cone 6 pots is a safer way to go.

[TonyC]

9 minutes ago, Min said:

Pinholes look worse at the bottom of the vase, I'm guessing this is an area you trimmed? Divots in the clay from trimmed areas in grogged clay can lead to pinholes. Run a rib over any trimmed areas to smooth the surface of the clay after trimming. The white glaze looks really stiff, could be it just needs to go a bit hotter. Really look over the glaze before firing, if you see any pinholes in the raw glaze then rub them over with your finger and get rid of them.

Crazing is from the glaze not fitting the claybody. Glaze has too high an expansion for the clay it's on. Since this is a commercial glaze it's difficult at best to fix it. Is this the clay you used? If so, it's rated to go to ^10 so I would check your vases for leaking. Just put some water in them and leave them sit on some newsprint for a few days. Cone 6 (midrange) clay for cone 6 pots is a safer way to go.

Thank you Min.  This is really helpful and insightful advice.   I never did check the pots beforehand for the pitting.  I will do as you say and rub the glaze as needed.   I have never heard or read about the trimming issue before - thank you.   As for the white matte, it is probably better suited at cone 6, but I was also firing some matte  shinos which I know fire better (less glossy) at 5.   

Yes, your link is the clay that I used.   I have used it before with some saggar fired pieces which have a very different requirement as no glaze is used.   I liked working with the material, so I ordered more.  The clay is listed as a 5/10 glaze which must mean that it is more 'vitrified(?)' at the higher temp than the lower, although it must be ok.   I have also used Laguna Bmix5 in the past which I liked as well.   Maybe that is a better clay for the glazed pots.  Any others you work with and would recommend (I live in MA)? 

Basically, I think I am thanking you for each of your points.  Very grateful.

 

[Min]

18 hours ago, TonyC said:

The clay is listed as a 5/10 glaze which must mean that it is more 'vitrified(?)' at the higher temp than the lower, although it must be ok.   I have also used Laguna Bmix5 in the past which I liked as well.   Maybe that is a better clay for the glazed pots.  Any others you work with and would recommend (I live in MA)? 

There has been a lot of discussion on this forum about these broad firing range claybodies. Consensus is it can't be as mature and tight as it can be at cone 6 and not be overfired at cone 10 or conversely if it needs to go to cone 10 to be mature it will be immature at cone 6. In all likelihood it will be immature at cone 6. You could do some absorption tests to confirm this or just the simple water in a pot on newsprint check. If after a few days the paper is wrinkled or damp then your pot is leaking. (even with no glaze the clay shouldn't weep) Your cone 5 B-Mix would be a better choice for working at cone 5/6 with. (it can go to cone 6 no problem). I'm not the best person to ask for claybody recommendations as I don't live in your part of the world, I'ld post the question in another thread giving your location and brands you have access to.

 

[TonyC]

2 hours ago, Min said:

There has been a lot of discussion on this forum about these broad firing range claybodies. Consensus is it can't be as mature and tight as it can be at cone 6 and not be overfired at cone 10 or conversely if it needs to go to cone 10 to be mature it will be immature at cone 6. In all likelihood it will be immature at cone 6. You could do some absorption tests to confirm this or just the simple water in a pot on newsprint check. If after a few days the paper is wrinkled or damp then your pot is leaking. 

 

This does seem to make a lot of sense.   As I mentioned, I got used to the clay for pit firing, but those pots are strictly for aesthetics.   Thanks for the sound advice.