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Crazing
Sept 22, 2015 12:31:02 GMT -6
Post by Arnie Benton on Sept 22, 2015 12:31:02 GMT -6
Here are 2 bowls, exactly the same clay, glaze, glaze loading, kiln firing - only difference is that one had a coating of an engobe with the same clay and 5%Ti and 2% whiting applied in bisque firing. No engobe -  With engobe -  The engobe completely eliminates crazing, makes more crystals and deepens the ground color. The glaze contains 4% Ti - so, is the engobe changing the glaze/clay interface in some way that just Ti in the glaze does not? Is the whiting part of the answer? More testing needed. Arnie |
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Crazing
Sept 22, 2015 16:53:00 GMT -6
Post by tileman2 on Sept 22, 2015 16:53:00 GMT -6
Arnie:
Interesting set of circumstances, and even more interesting differences. By TiO2 I assume you mean rutile?. I see the classic colors associated with rutile primarily in the second piece. Then green in the second piece reminds me of ilmenite. The first piece has the classic whitish/grey color in the center of the piece. The second piece has the milky/greyish coloration at 10 o'clock, 2-3 o'clock, and again at 5-6 o'clock. I have seen these patches before when high levels of TiO2 are used on horizontal pieces. I would vote TiO2 before I would whiting- personal opinion.
The more interesting occurrence is the crystal population on the engobe piece. Which makes me wonder if it is more mechanical than chemical. The colorants ran and pooled in the center on the first piece; but remained evenly dispersed in the second piece. The dramatic difference in field color also raises questions, just as much as crystal population. In engineering there is a term called "friction", which is resistance to flow. The first piece seem to pool colorants and TiO2 readily, the second piece remained evenly dispersed. So I will raise a third question: did the engobe create mechanical friction that restricted the natural flow of the glaze towards the center? Great looking piece regardless the cause.
Tom
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Crazing
Sept 22, 2015 21:05:34 GMT -6
Post by Arnie Benton on Sept 22, 2015 21:05:34 GMT -6
Hi Tom -
There's no Rutile or Ilmenite in the glaze or the engobe. There's Fe Mn Co and Cu in the glaze along with the Ti.
I agree with you about the white patches - I get lots of them with a Ru Ilmenite glaze.
Your observation about friction is a very good possibility and one I never noticed - you are extremely observant and I appreciate it.
I brush the engobe on and looking right now at my unglazed bowls with the engobe, some of them do have irregularities.
So I have to try the engobe without the Whiting and smooth the surfaces.
Arnie
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Crazing
Sept 23, 2015 9:24:24 GMT -6
Post by Arnie Benton on Sept 23, 2015 9:24:24 GMT -6
What a difference 24 hours makes - I went into the studio this morning and -  So, no miracle engobe - what a shocker. I have to regroup from this disappointment. Arnie |
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Crazing
Sept 23, 2015 18:57:36 GMT -6
Post by tileman2 on Sept 23, 2015 18:57:36 GMT -6
Arnie:
I have scrolled back and forth looking for any signs- crazing, clay/glaze interface checking. I do not see one thing on the finished piece that would give any indication this was coming. Baffles me, at best I would be guessing at the clay body because I do not see anything in the glaze that indicates a problem. Sorry- beautiful piece. Engobes are thin applications- and the same clay to boot: so that would be off my list of considerations as well. The glaze broke uniformly with the clay- so you had a good glaze bond and uniform thickness. Stumped!!
Tom
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Crazing
Sept 24, 2015 9:47:28 GMT -6
Post by Arnie Benton on Sept 24, 2015 9:47:28 GMT -6
I think you're right ,Tom - that's why it came as such a shock to me. I put the piece into a striking firing - might as well get some more information from it -  I continue to be amazed by what "just" heating pots to 1350 degrees F can do. Arnie |
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Crazing
Sept 24, 2015 19:53:32 GMT -6
Post by tileman2 on Sept 24, 2015 19:53:32 GMT -6
Arnie:
Might have an answer for you; at least a possible explanation. Fairly involved with my supplier in St. Louis and we have been emailing back and forth about changes and problems with materials in the last few months.
Pending the clay recipe: custer feldspar can account for 17 up to 25 percent of the clay body. So this could affect vitrification or lack thereof. Have heard complaints about zinc, silica, and ferro frit 3110 changing or acting differently over the past few months. The changes in custer feldspar might be creating new problems- just a thought.
Tom
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Crazing
Sept 24, 2015 20:26:09 GMT -6
Post by jerrysawitz on Sept 24, 2015 20:26:09 GMT -6
i was having the same problem so I asked John Tilton and he said slow down the refire especially around quartz inversion. haven't had an issue since.
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Crazing
Sept 24, 2015 21:20:37 GMT -6
Post by Arnie Benton on Sept 24, 2015 21:20:37 GMT -6
Both Tom and Jerry are probably on the right track. The clay does have a Custer substitute, and I was slowing down cooling from 1200 down to 1000 and from 600 down to 400, but I stopped doing that for the past 4 or 5 firings - I thought I'd found 'the answer' by putting Silica sand under the pieces.
Over the years I've had breakage with many clays, especially with larger plates and bowls - over 12 inches or so. I attributed many of these to poor throwing technique or uneven drying - in the future I'll wait 24 hours before declaring 'victory'.
Arnie
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Crazing
Sept 25, 2015 17:10:30 GMT -6
Post by tileman2 on Sept 25, 2015 17:10:30 GMT -6
Arnie:
I have followed your posts closely in this and the old forum. There was a continuous discussion about crazing for several months running. After you started using silica under your plates that problem disappeared. The pieces posted after you began that habit did not show any of the crazing or checking that was present before hand. The piece is moving freely even if linear expansion were present during the inversion temps: movement during bisque firing is much more prevalent during that firing than during glaze firing. I am not dismissing linear expansion during inversion temps; but I am saying the piece does not show the classic signs of crazing or clay/glaze interface checking. If that much expansion did occur on the piece shown; the glaze would have reflected the problem before it split. From all the information thus far, I would put the clay body at the top of the culprit list due to changes in the custer feldspar/flux. The glassy matrix within the body that should have formed during vitrification obviously did not. Now for a final (maybe) question: was this piece thrown from a new box/batch of clay? As far as your throwing technique being in question- if that were truly the case you would have more than just one piece split. Finally, if uneven drying of the greenware was in question- the piece would of blew apart during bisque firing.
Tom
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Crazing
Sept 25, 2015 20:46:25 GMT -6
Post by jerrysawitz on Sept 25, 2015 20:46:25 GMT -6
just differences in bottom and wall thickness can cause all kinds of problems. especially with plates. i had some split like Arnie well after the fire. since i slowed the refire, as in strike or reduction i haven't had one crack since.
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Crazing
Sept 25, 2015 23:19:32 GMT -6
Post by evan cornish-keefe on Sept 25, 2015 23:19:32 GMT -6
What type of clay body are you using?
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timhanrahan
New Member
Still growing crystals!
Posts: 22
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Crazing
Sept 26, 2015 9:31:05 GMT -6
Post by timhanrahan on Sept 26, 2015 9:31:05 GMT -6
Arnie,
All other comments aside, did you put the engobe on top and bottom of the platter? If not, the differential expansion/contraction may be contributing to a shearing factor. I have seen issues with incompatible liner and cover glazes. My $0.02.
Tim
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Crazing
Sept 26, 2015 14:02:22 GMT -6
Post by tileman2 on Sept 26, 2015 14:02:22 GMT -6
Arnie:
Glue it back together, glaze it black and orange and I will take it out to the trap range.
Tom
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Crazing
Sept 26, 2015 16:09:01 GMT -6
Post by Arnie Benton on Sept 26, 2015 16:09:01 GMT -6
Jerry - most of my plates and bowl have cracked in a circular fashion, at the junction of foot and wall - I have attributed it to making that junction too thin. Other throwing errors have led to slumping.
Evan - my clay body is a cone 10 porcelain, custom made and the recipe was changed because of the loss of Custer Spar - the person who makes and uses it isn't having problems - but he single fires! The above bowl that cracked was bisqued and then fired. I'm going to go through my records and figure out what percentage of bowls are breaking -
Tim - No, I haven't been putting the engobe on the bottom - that's a good idea. Since the engobe is mostly the same clay as the piece I haven't bothered to do the bottoms, or even the sides.
Tom - my wife is gluing it together! - a few years ago I was complaining about running out of shelf space and one of my kids suggested using them for skeet practice.
The kiln is cooling now - all the pieces are on sand and I've put cooling from 600 down to 400 at 40 degrees F in the schedule. My kiln has lots of insulation and is cooling at about 75 degrees an hour from 1200 down to 1000. Maybe I will need to slow that down as well. None of the pieces have the engobe, and one piece is being fired for the sixth(!) time.
I'm glad we're having this discussion - sure helps me.
Arnie
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