Can you predict the approximate drop/hold temperature in a drop and hold firing from the chemistry of a glaze?

[Marilyn T]

Min:   your suggested firing schedule resulted in an improvement.  I now have two without pinholes or dimples; one with pinholes; and the rest with dimples.  Thank you.   

# 1 Original firing schedule where the Variegated Blue              # 2  Firing schedule where the refire and studio white had no defects

and Rutile Green had no defects but the rest did was:                             ^5 SS bent to shelf; ^6 slight bend.  = 5.5

100.     220.     30                                                                                                     100     220.     30

350.     2000.    0                                                                                                       350    2000.    0

108.     2190.     5                                                                                                       108    2165.    18

9999.   2090.    10                                                                                                   9999.  2085.    12                       

500.      1900.     10                                                                                                  500.     1900.    10

125.       1400.      0                                                                                                   125.      1400.    off

The results of the glazes for Firing # 2 on 1 lb. bowls were:

Wollastonite Clear -- 1 pinhole outside 3 on rim

Studio White -- no defects

Folk Art White -- dimples

Variegated Blue -- dimples

Rutile Green -- dimples

Refire -- no defects

Test Glaze on test tile -- four pinholes on side next to element.  

I also fired a commericial glaze (Amaco HF-9) at cone 5 + 5 minutes but it had dimples and one large pinhole (^5 bent to shelf).  Two potters on instagram fire this clay at 5+ and seem to have good results with commercial glazes.  One uses Amaco Honey Flux (which is a fluid glaze).  Amaco says some of their glazes shiver on this clay.  

I welcome all suggestions that might result in a defect free firing schedule for this clay.  

Neil:  I will try your suggestions on shortening the bisque schedule in my next endeavour.  Right now I have over a 100 pieces bisqued using the schedule I outlined above, so will concentrate on the glaze firing schedule with the already bisqued pieces.  

Peter:  I don't think it is the wollastonite in the glaze that is the problem (one of the glazes has Wollastonite and the other Whiting), however there may be Wollastonite in the body.  I sieved the clay slurry and did find some small white chunks in it that didn't go through the sieve.  They may be silica or feldspar??  Photo attached.   On one of the bowls that I glazed in a commercial clear, there was a white chunk on the outside, under the glaze that was the same as the one on the screen.  

Bill:  Hopefully the problem will be solved with the firing schedule, so I won't have to change the glaze fluidity (not my strength).  I'm thinking your suggestion of multiple holds may be part of the solution.  

Hulk:  Thanks for your suggestions.  I still hope to find a solution without changing the clay.  

 

 

Edited February 21 by Marilyn T

[Min]

If the dimples are from unhealed pinholes then it's logical to think that giving the glaze more heatwork will smooth them out. It's interesting that the refired bowl smoothed out the pinholes and the clay didn't bloat. What happened when you fired the Wollastonite Clear to a full cone 6, was the clay bloating or had a higher absorption? At ^7 you had increased absorption, correct?

Common fix to make the glaze more fluid would be to remove some alumina and silica but keep the alumina:silica ratio above 1:10 but that's going to mess with your COE and you might then get crazing which in turn would need some of the higher expansion sodium and potassium being replaced with a lower expansion flux such as additional magnesium or lithia, at this point so much tinkering will have been done that it will basically be a different glaze.  I can alter your recipe to do this if you want to give it a go. Do you have Ferro 3110?

Do you have the recipe for the Studio White? Have you noticed if different thicknesses of the Wollastonite Clear results in more or less pinholes/blisters? I also think the pinhole issue is from the claybody but understand you are invested in using this body for now so it looks like glaze formula + firing + glaze fluidity + glaze thickness are all going to play into trying to overcome it.

[Marilyn T]

What happened when you fired the Wollastonite Clear to a full cone 6, was the clay bloating or had a higher absorption? Haven't done this yet but the refire didn't show any signs of bloating.  Previous tests with other glazes at higher temperature (Glossy Base Glaze Liner and G2936B) also didn't show signs of bloating, but I did get warping on the 25 hour NC firing.  

At ^7 you had increased absorption, correct?  Yes it went from 0% at ^6 to 6.6 at ^7.  

 I can alter your recipe to do this if you want to give it a go. It's worth a try.  Thanks I'd appreciate it.   Do you have Ferro 3110?  I can get some.  Presently I have Ferro Frit 3134, 3124, 3195 and 3249.  

Do you have the recipe for the Studio White?

• Minspar 200. 47.378
• Silica 20.247
• Gerstley Borate 13.875
• Whiting 8.428
• Dolomite 6.064
• Zinc oxide 4.008
• Bentonite 2.0
• (Tin Oxide 5.0)

Have you noticed if different thicknesses of the Wollastonite Clear results in more or less pinholes/blisters?  I haven't experimented with different SG's yet with this glaze.  Tony recommended 1.4 for the G2936B so I kept that consistent for the white glazes.  The FAW (1.47) VB (1.43) and RG (1.43) have higher SG. The commercial clear has a SG of 1.45).  There hasn't been more pinholes in the bottom of the bowls (where it  might be thicker).  Most of the original testing (with GBGL and G2936B) had pinholes clustered outside near the elements.  At some point in testing  that switched to none on the outside and some on the inside.  Now it doesn't seem to have a pattern.  

I also think the pinhole issue is from the claybody but understand you are invested in using this body for now so it looks like glaze formula + firing + glaze fluidity + glaze thickness are all going to play into trying to overcome it.  I agree.  Good learning experience but it would be nice to have a final product!!

[Min]

Firing to a full cone 6 is worth doing. This might just be enough to rid the glaze of the dimples.

I'll rework the recipes so you have an early melting one and a late melting one, this way if the glaze is sealing off the clay before it's finished offgassing there might be a difference in how well they heal over the pinholes. I'll use the Studio White as the base for a late melting one, zinc melts later in the fire and as it's an auxillary flux in the Studio White it might work to do a modified version of that one. Downside is zinc itself can create problems with pinholes so it might be counterproductive. What is it about the Studio White that you don't care for it as much as Wollastonite Clear?

If you have any flat test pieces you could use as tests I would use them, pinholes are harder to fix on flat surfaces.

[Marilyn T]

Thanks Min.   I'll do a  full cone 6 fire on a small bowl as soon as I bisque some test bowls.  I'll also start throwing some small plates to use as testers.  

What is it about the Studio White that you don't care for it as much as Wollastonite Clear?   I only have about  7 L of Gerstley Borate left and was not sure how Gillespie borate would affect the glaze so thought Wollastonite clear would be better in the long run.  

I chose Studio White, not only because it was a proven glaze, but due to  the Silica being the only common material and I therefore thought it would be easier to test the firing schedule as the source of the pinholes.  It also has a high LOI compared to the Wollastonite clear so thought I could eliminate the gas from the glaze materials as being the source of the pinholes if they both performed the same (i.e. equal # of pinholes).  

I'm looking forward to trying out your reworked recipes and appreciate the time you are donating to help me work toward a solution for this problem.    

 

[Min]

I've altered the Studio White to remove the Gerstley Borate and supply the boron with frits instead. I've also removed the Minspar 200, I did this because I wanted to get some clay into the recipe and the original only had 2 bentonite and zero epk, most of the sodium and potassium is now supplied by a frit. I replaced the calcium carbonate / whiting with wollastonite for two reasons, firstly it melts better than whiting + silica and secondly it has less loss on ignition. I kept the oxides the same in the formula, they are just being supplied from different materials. My hunch is it will melt more than the original. Kept the same flux ratio and reduced the LOI. Because of all the frit this glaze should have 2 main melting zones, lower temp for the frits to get started melting and then higher temp for the zinc. (put the epk in your mixing container first so the frits don't just stick to the bottom)

For the Wollastonite Clear I used 3 frits, I kept the boron level high. I wanted to reduce the silica and alumina amounts plus raise the silica above a 1:10 alumina : silica ratio. I reduced the sodium and potassium and increased the magnesium. By reducing the alumina and silica this version should be more fluid than the original, it did raise the COE slightly though.

Overall the Wollastonite Clear should make a more durable glaze than the Studio White. I didn't add tin to any of the recipes, wanted to see the numbers without it. (use 325 mesh silica) Please ask if you have any questions about why I subbed what I did if you are unsure of anything. (as always please test a small amount of the altered versions, what looks okay on paper doesn't always translate to a pot)

edit:  "I'll do a  full cone 6 fire on a small bowl as soon as I bisque some test bowls." +1

 

 

 

 

Edited February 21 by Min

[Hulk]

Be sure to carefully check for application defects!
This may be what was nagging at my memory?
During yesterday's glaze session, while checking for tiny bubble marks and pinholes, an "ooh yeaa" moment:
microtips - Studio Operations and Making Work - Ceramic Arts Daily Community

Where any air can be trapped in the clay, or at the edge of applied underglaze, or at the edge of wax resist, et cetera, as the glaze dries, then the bubble pops - there's a pinhole!
I'm checking the glaze layers more carefully than ever!

 

[Marilyn T]

Update for anyone following this thread:

 I have now finished my firing schedule experiments and didn't come up with a schedule that worked well.  So far there are  two possible solutions:  (1) refire and (2) only glaze one side (to allow gases to escape from the unglazed side). I will use two firing schedules:  my original firing schedule (the one that gave good results on Rutile Green, and Variegated Blue) and Min's suggested firing schedule which gave a good results for Studio White for the next set of experiments --  thickness of application and adjusted glaze chemistry.   

The results for the over 20+ firings are:

1. peak temperature:  the best results were in the range  2165-2190.  I used both the 108 ramp and 27 degree ramp.  Worse results were experienced going higher or lower than this range.  
2. soaks at peak:  increasing the length of a hold from  2190: 5 - 30 minutes ; 2165: 18 to 30; and 2133: 30 to 75  didn't improve the results.
3. slow cool rate:  tried 500, 125 and 60 and got worse results the slower it went.  The VB developed blisters with the extended hold.  
4. drop and hold soaks:  soaks in the range  1900 to 2090 (going down by 10 degrees) didn't make any difference in the results for the Wollastonite Clear or Studio White.  The only success on the VB and RG were in the original firing schedule.  

Next predicted update in one month.  

[Marilyn T]

Update # 2 for anyone following this thread:  Results on Wollastonite Clear adapted  glaze.

Application:  Varying the SG from 1.40 - 1.46 did not eliminate or make the problem better.  I like the look of the lower SG and it has the added advantage of making it easier for the gases to escape.  The addition of epsom salts to increase the thixotropy also didn't result in an improvement.  

Defect:  more pinholes but smaller.  (see photo 2X).  I put India ink on the ware and viewed it through a jewellers loupe, because at first glance  it didn't look too bad (no glasses; bad lighting)   

Causes of defect: (my theory)

•  gases from body have not been fully expelled due to early melting of glaze which seals the surface (starts at 1800F);
•  clay vitrifies >2150 before all the feldspars are expelled (2044-2192).  
•  oversupply of Calcium which has high surface tension (fluxes at 2012) and results in a glaze that is not fluid enough to fill the pinholes.  

Next steps:

1.   try a glaze that has later melters and fluxes which lower surface tension.    I will be trying the Studio White adapted recipe that Min sent me, as well as a similar recipe G3806 C and 3806 E from Digitalfire.  They have lower B2O3 (0.10 to 0.15)  and use Zinc oxide.  (Zn limit 0. - 0.2 and the recipes call  for 0.12 - 0.14 so shouldn't cause pinholes).  

All comments/suggestions/recipes welcome.

Next update in a couple of weeks when I've finished testing the Studio white glaze.   

[Min]

It seems like you are going through an inordinate amount of glaze testing to solve what really appears to be a clay problem. Have you tried contacting Laguna with the batch number and asking them if they have any thoughts on this?

Is using a different red clay an option? @Callie Beller Diesel uses Plainsman M390 very successfully, is this an option for you? Could you use the clay you bought for another purpose?

[Marilyn T]

I have attempted to contact Laguna twice before about a different clay (Laguna 65) and received a reply that they were forwarding my question to someone else -- never heard back.   So I probably won't contact them again.  

I recently purchased a box of M390 and threw some test bowls.  I am including them in my next series of tests to see what the results are -- figured if Tony had success on M390 with the glaze G 3806 that I could use that as a comparison for the results on the Laguna 613 and go from there.  He uses the C6DHSC firing schedule so if I don't have good results on the  M390 it will indicate that something else is going on that I haven't thought about.   As part of covering all the bases I'm bisquing a few bowls at ^02 for comparison. Going to give the Laguna clay about another 2 weeks to reveal its mysteries then abandon ship (I only have 6 bags left).   -- 6 more glaze  firings to go (the golf course is calling!)  

The good thing is that I am really good at throwing test bowls now.    

Edited March 28 by Marilyn T

[Min]

56 minutes ago, Marilyn T said:

I recently purchased a box of M390 and threw some test bowls.

Tagging @Callie Beller Diesel on this in case I'm misremembering but I believe she fires 390 to cone 7 for absorption reasons.

[Callie Beller Diesel]

On 3/27/2024 at 10:06 AM, Min said:

It seems like you are going through an inordinate amount of glaze testing to solve what really appears to be a clay problem

This. 100000X this. If your clay body is giving pinholes of some kind with several glazes, it’s the clay body, not the glaze or the assorted firing cycles. It may well be that an ingredient in the Laguna body just doesn’t want to play nice, whatever is done with it. You have to weigh wether you like using this clay more than you want a perfectly defect free surface.

If you need a good red clay/white glaze combo, M390 and Ravenscrag slip 80/20  with frit 3134, and 2 opacifiers of your choice. (I recommend a tin and zircopax combo if you don’t want any titanium variegation). Reasoning: they give different qualities of white. Tin by itself is very soft, and can be grey-ish or even drift towards a violet in certain light when used on M390 in particular. It’s also expensive. Zircopax by itself is very stark, and gives a very “bathroom fixture” white that resists breaking or pooling. The 2 together give a soft white without weird tonal casts, gives a little more visual interest and offsets the end price of the glaze. I use a max of 10% opacifier, but judging from the look of your photos, you might want a slightly more translucent version, so you could see what happens with 7 or 8%. 

I fire with a very analog kiln at home, so I can’t give specific numbers on ramps and speed. I slow my kiln down around the carbon burnout stages mentioned in the Steve Davis article, and I judge that through the colour of the kiln. I’ve recently started working at a place that has brand new Bartlett 2 touchscreens though. I feel spoiled! They are to be preferred over my methods.

I think speed of bisque is more important than end temperature, so slow through the appropriate zones. I think the Naomi Clement one is a bit of overkill.  The 12-14 hours of Davis’s cycle (assuming no drying soak at the beginning) is plenty long. Note the pauses around 1500F that both Clement and Hulk have suggested.

The glaze cycle I use to get what is effectively cone 7 is a drop and hold, similar to what you’ve already worked out. I go to cone 6 with a 20 minute hold after the drop.

Note that there will be upcoming changes to M390: according to my supplier, Plainsman demand has gone through the roof lately, and they’re struggling to keep up. They have one clay mixer that they have to cycle all their different clay bodies through from lightest to darkest. While another one has been ordered, it’s not expected in until next year. In the meantime, they’ve made the decision to remove the 80 mesh sieve from the machinery in order to get the clay to go through faster. This will affect M390’s texture starting this year. So we’ll all have to revise some porosity testing until they get the new machinery in place.

 

The last suggestion I would have is another bisque cycle. But this time, only go to 010. There have been a few reliable potters in various internet spaces lately speaking about this (For Flux Sake podcast in particular). I believe the argument is that the application a more porous clay body gives a dipped glaze results in fewer defects. I haven’t experimented with this personally though, so that one is a shot in the dark if nothing else works.

[Marilyn T]

Thanks for your reply Callie.  I do like the clay but a defect free surface tops that.  My current testing is to establish the best possible results for the Laguna 613 dinnerware set before committing to either refiring it to get a smooth white surface, or to glazing it with Rutile Green or Variegated Blue and using it in my own kitchen.   I will add your suggestions to my list of firings as I continue to add to my knowledge base.  

With regard to the M390 glaze,  I accidentally calcined the Ravenscrag instead of roasting  it -- but from what I read it shouldn't affect the glaze -- it broke up fairly easily.  Thanks for the heads up on the temporary change in texture of the M390.  I did find it coarser than the Laguna 613 so hope it is smooth enough to allow rims to be unglazed -- if that is the chosen design.  I purchased the box of  M390 this month,  so not sure if it is a minus 80 mesh sieve batch or not. I'll see what my absorbency tests reveal.  

Thanks again for generously sharing your ideas.  

 

[Callie Beller Diesel]

Usually M390 is a VERY smooth clay. So if you’re finding it’s got some texture, it may well be the 60 mesh version. My supplier brought up the possibility back in January.