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KitchinSarah

Laguna 65 And Crazing

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Hi there, 

 

I have been using Laguna 65 for years now and have enjoyed throwing with it and love the color once fired. I have been having serious issues with crazing from the beginning, and now with my new surface designs I would really prefer a good glaze fit. I have tried several different clears on this clay body including commercial Amaco HF-9 and several clears that I mixed on my own. ALL OF THEM have crazing. Normally delayed crazing. Of course this indicates that there is a poor fit with the glaze. But after all of the glazes I have tested I am wondering if there is something I am missing with the clay body itself? I fire to cone 6. 

 

I was wondering if anyone else uses 65 and has either A)found a clear glaze that works successfully for them or B) has suggestions for what I might be able to do to solve my issue.  

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Looking at the data on the clay it has a coefficient thermal expansion of 4.8 that seems quite low. Not surprised most glazes you have tried craze. I am sure my glazes are around 6-7 which seem to fit my clay.

 

"Glazes that have a higher expansion than the body by implication also contract more on cooling. This puts the glaze under tension, stretching it, sort of a "size 6 mug in a size 5 glaze" situation."

 

I am sure most cone 6 transparent I have looked at are around the 6 mark too. Getting a small bit of lithium feldspar into a glaze or any source of lithium oxide helps to lower expansion. I will have a quick look at recipes and see if I can find one that looks a good starting place. Found this http://digitalfire.com/4sight/recipes/cone_6_ultraclear_glaze_for_porcelains_106.html but even the ultra low expansion glazes seem to be around the 6, nowhere near 4.8.

 

Why do they say it is good for functional whiteware? Feels like I am missing something but right now a glaze that would not craze seems difficult to achieve. All the porcelains I had a quick look at were above 5 thermal expansion. I would try ringing the people like oldlady says and see if they do have a glaze that will fit the clay.

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A couple to try if you haven't already. My favorite low COE clear, uses a magnesium frit to help keep the coe down. Sub Minspar for F4 if you don't have any F4 (not available any more). If you use this glaze it needs to be fairly thick for smooth coverage. Doesn't work with chrome/tin pinks and red stains but otherwise it's great.

 

G1215U ^6

EPK  14

Silica  26

Wollastonite 14.5

F-4 spar  24

Ferro 3249  20

total: 98.5

plus macaloid  1  (or 2 bentonite)

coe is 5.76 using Insight

 

This one uses lithia to help with the lower coe plus it has magnesia (from the dolomite). I haven't used this one so no clue how it behaves.

 

Robust E Clear ^6 

EP Kaolin 18.20
Silica 31.80
Spodumene 9.10

Dolomite 10.00
Strontium Carbonate 1.80
Ferro Frit 3195 27.30
Zinc Oxide 1.80
total 100.00

plus bentonite  2

coe is 5.54 using Insight

 

If you are looking for a commercial clear Laguna MS - 29 Clear Bright might work. It's expensive and hardpans in seconds, (the dry version) so add some macaloid or bentonite to it. I haven't used this in a number of years but it used to fit my low coe clay, hopefully it hasn't changed.

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Still sounds to me like those values are too high and will still craze. How do you ever get it down below 4.8 :unsure:

I believe those two glazes are made more elastic by the fairly high boron content, even though the coe isn't super low they might still fit without crazing. only one way to find out....

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Still sounds to me like those values are too high and will still craze. How do you ever get it down below 4.8  :unsure:

 I am not sure you can use the COE of the clay as the guideline for crazing.  I use a Laguna clay with a COE of 4.64 and have no crazing problems.  Jim Robison wrote an article about Clay in Studio Potter volume 9 number 2 June 1981. I had the good fortune to attend one of his workshops where He gave out 9 recipe's so you can test clays to see what COE of glazes will work on a clay body.  He tested from COE 5.29 to 8.11.  I line blend from COE 5.05 to 8.33  on  every clay body that I am thinking about using.

David

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Guest JBaymore

There are a couple of different approaches to calculating theoretical COtE in glazes.  They give a bit different numbers. 

 

Where did you get the number for the body? (Which has to be from dilatometer... you can't calculate that number mathematically for non-glassy phase materials -yet-.)  That is a somewhat low expansion body (if that number is accurate).

 

What method are you using for the COtE in Insight for glazes?  And remember that the calculated (as opposed to measured) COtE for a glaze ASSUMES that the glaze is at a fully glassy phase state.  If anything precipitates out as micro-crystalline materials upon cooling....... the calculated COtE number will be off a bit.  This has a huge impact in a lot of "studio potter" type glazes.

 

Another factor to keep in mind with all of this stuff is where are you getting your materials data for the glaze calc program?  Are you using the "stock" MDT tables that are installed in it.... or are you updating with YOUR supplier's "typical analysis" sheets?  The slick looking software (of any kind) with all those pretty numbers is only as good as the DATA that it is working with.  (Old computer term... garbage in-garbage out.)  Even with the best info you can easily get (current typical analysis sheets), the data is only 'close approximations' (at best) of what is in those bags you are weighing your glaze batches from.

 

We can get all caught up in what looks like precise science....... but such software is only ONE tool in the ceramist's toolbox to deal with this stuff.  Gets you in the right ballpark.  Test, test, test.

 

The best method to deal with this issue is to use 'the glaze series that has a steadily increasing COtE' method that David mentions above.  (I have my students do this in the materials course every semester.)

 

And yes, call the supplier to check their thoughts.

 

best,

 

......................john

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I did a quick google search for Laguna 65 and their webpage has the COE http://www.lagunaclay.com/clays/northeastern/wc609.php Did a quick search through some of their other clays to see what sort of area they were in.

 

Even if all the estimates for glaze expansion are off and we all have terrible materials data, 4.8 seems across the other side of town and nowhere near the ballparks most glazes are sitting in. Even looking at porcelains that are said to be lower expansion bodies they don't seem to come in under a 5.

 

I had assumed their number is accurate with using a dilatometer and the OP saying all glazes are crazing. To theoretically get something under 4.8 seems rather impossible, maybe in reality it is feasible with some testing.

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why not contact laguna and ask their tech people for a glaze?

 

All the clay, all the casting slips, and 80% of the glazes I use are Laguna. I've contacted them several times over the past 18 months with questions and I'm sad to say that they don't support their products. The first couple of times they asked me about my witness cones and I hadn't used them. So I started using witness cones. Now the conversation goes: "Well, what did your witness cone do?" "It fired properly at ^5 on all middle shelves. Top shelf was ^4 and bottom shelf was ^6." The answer is: "Well not all glazes fit all clay bodies." 

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what is it about that particular clay that makes you want to continue using it?  yes, you like the fired color but it isn't the only available clay to fire to that color.  if it is causing this much trouble, it might be time to toss it.

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I recently switched to Laguna 65 and am currently working on developing glazes to fit this body.  I've  had promising results by modifying the Glossy Base Glazes 1 & 2 from Mastering Cone 6 Glazes.  I also had good results with a glaze that was a modification of Digitalfire's G1215U which I list below.  I modified the glazes to have an expansion coefficient just above 6.

I've noticed there seems to be a significant difference between calculated and measured coefficients of expansion of glazes with the measured COE usually being lower than the calculated.  My feeling is a glaze to fit  this body should have a calculated COE of around 6 x 10-6.     

 

Here's the recipe for the glossy base glaze I had the best results with.        

Kona F4 - 22

Ferro Frit 3249 - 12

Ferro Frit 3134 - 8

Wollastonite - 12

Talc - 5

EPK - 15

Flint - 26

 

This glaze calculated to a COE of 6.04 and the fit seems to be good.  I've cycled several pieces multiple times between a 300 degree oven and a bucket of water with no adverse effect.  

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I use this one in my studio, and it fits everything. It's from Digitalfire, but I modified it slightly to melt a little more for better clarity.

 

G1214W

Frit 3134     26.08

Neph Sy     11.07

Whiting       7.98

EPK            25.11

Flint            29.75

 

COE is 5.37

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I use this one in my studio, and it fits everything. It's from Digitalfire, but I modified it slightly to melt a little more for better clarity.

 

G1214W

Frit 3134     26.08

Neph Sy     11.07

Whiting       7.98

EPK            25.11

Flint            29.75

 

COE is 5.37

 

Which program do you use to calculate COE? I plunked it into Insight and the coe comes up at 6.69 which I thought kinda makes sense with nepsy, whiting and that much 3134 in the recipe. Wonder where the oopsies is? Curious. (I am just using stock MDT materials figures with Insight)

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John:

Ron Roy and I had a fairly lengthy conversation about COE and material data. He uses Insight, but has imported his own data sheets that are current (2013-2014). Being a retired clay guy: they had every batch of raw materials analyzed before mixing began.He was kind enough to give me his most recent specs on the most commonly used clays and raw materials. Obviously the glaring one was custer feldspar which use to be 10.08% potassium; but now runs as low as just over 5% up to 7.57%. (his testing). Gerstley Borate jumps around alot as well. Ron also showed me a few graphs of the actual COE as tested by his dilatometer for several popular glazes: only one of the four actually fired close to the projected COE. Speaking only for me: in the future when a program estimates a COE: think I will trim down a little so that it is more of a fit.

Nerd

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I use this one in my studio, and it fits everything. It's from Digitalfire, but I modified it slightly to melt a little more for better clarity.

 

G1214W

Frit 3134     26.08

Neph Sy     11.07

Whiting       7.98

EPK            25.11

Flint            29.75

 

COE is 5.37

 

Which program do you use to calculate COE? I plunked it into Insight and the coe comes up at 6.69 which I thought kinda makes sense with nepsy, whiting and that much 3134 in the recipe. Wonder where the oopsies is? Curious. (I am just using stock MDT materials figures with Insight)

 

 

I'm using Hyperglaze. I checked out my materials figures and they were virtually identical to the DigitalFire numbers, so obviously there's some differences in how they are calculating the COE. I ran the recipe through another program - one of the online calculators - and it put it at 5.83. Glaze Master puts it at 6.82. Lots of variation there. From what this is showing, and from what others have said, we get back to the old rule of test, test, test......

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That's interesting Neil. I didn't know the differences could be so great in how they calculate the COE. If the variables (materials data) was virtually identical I wonder where the math varies in the calculations.

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Guest JBaymore

John:

Ron Roy and I had a fairly lengthy conversation about COE and material data. He uses Insight, but has imported his own data sheets that are current (2013-2014). Being a retired clay guy: they had every batch of raw materials analyzed before mixing began.He was kind enough to give me his most recent specs on the most commonly used clays and raw materials. Obviously the glaring one was custer feldspar which use to be 10.08% potassium; but now runs as low as just over 5% up to 7.57%. (his testing). Gerstley Borate jumps around alot as well. Ron also showed me a few graphs of the actual COE as tested by his dilatometer for several popular glazes: only one of the four actually fired close to the projected COE. Speaking only for me: in the future when a program estimates a COE: think I will trim down a little so that it is more of a fit.

Nerd

 

 

Glazenerd,

 

I was talking to Ron yesterday about what you are doing with the crystals and the clay bodies.  He described some of the work you are doing and the two of us agreed that neither of us had ever seen one particular aspect of what you've gotten.  Cool stuff.  (I won't disclose that here in case you want to 'keep it under wraps' for now as you explore it.)  Can't wait to see where you take it. 

 

There are a number of ways to calculate COE...... the main thing is that it gets you 'in the ballpark'.  The dilatometer is the tool you really need to do that stuff...... bit they re not cheap.  Ron got his for a "song". 

 

Sorry you missed getting to NCECA itself.  Carty's presentation had some more good stuff he shared on toxicity and disposal, and Katz had a nice piece of new info on really durable cone 04 glazes.

 

Best,

 

................john

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JBaymore Posted Today, 12:30 AM

He described some of the work you are doing and the two of us agreed that neither of us had ever seen one particular aspect of what you've gotten.  Cool stuff.  (I won't disclose that here in case you want to 'keep it under wraps' for now as you explore it.)  Can't wait to see where you take it.

Thank you very much. Coming from the two of you, I cannot get any better compliments. I appreciate you keeping this propriety information under wraps. It is a theory I have been working on for several years.

Nerd

 

Edit note: In my construction years I attended several EPA courses on lead and asbestos abatement, which included handling, disposal, and safety. So I have that working knowledge: thermal decomposition/solidification is the only safe way to dispose of toxic materials.

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That's interesting Neil. I didn't know the differences could be so great in how they calculate the COE. If the variables (materials data) was virtually identical I wonder where the math varies in the calculations.

 

I did try putting in different materials data from a couple of charts, and it didn't really change all that much- not to the point that I got close to your numbers- so there's definitely something going on with the math. The good thing is that I know that my glaze fits my clay bodies, so if I can find glazes that have a similar COE using my software, then I'm good to go regardless if the numbers are 'correct' or not.

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That's interesting Neil. I didn't know the differences could be so great in how they calculate the COE. If the variables (materials data) was virtually identical I wonder where the math varies in the calculations.

 

I did try putting in different materials data from a couple of charts, and it didn't really change all that much- not to the point that I got close to your numbers- so there's definitely something going on with the math. The good thing is that I know that my glaze fits my clay bodies, so if I can find glazes that have a similar COE using my software, then I'm good to go regardless if the numbers are 'correct' or not.

 

 

Yeah, thats the conclusion I came to too. I'm thinking that for glaze recipe sharing it would be good to know which programs have what idiosyncrasies so that ballpark figures could have more meaning. 

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