A high-nepheline, zero-silica cone 8 silky matte glaze is cutlery marking and crazing. Why?
I will show you how found a recipe on Facebook, assessed it, substituted my own materials, tested it, adjusted it. Now it is like a cone 10 dolomite matte.
How I found a recipe on Facebook, substituted a frit for the Gerstley Borate and added the extra silica it needed to fight crazing. I got a fabulous cone 6 clear.
Using Insight-live I will demonstrate the surprising amount of silica some cone 6 base glazes that appear OK will accept and still melt well.
How to spot out-of-balance indicators in the chemistry of glazes that suggest susceptibility to scratching or cutlery marking.
Raw lithium carbonate can be replaced with a lithium-containing frit if you can do the chemistry. And you can at insight-live.com.
Use Insight-Live.com to do major surgery on a feldspar saturated cone 10R glaze recipe with multiple issues: blistering, pinholing, crazing, settling, dusting and possibly leaching!
We will substitute wollastonite for whiting and a frit for Gerstley borate in G2571A while maintaining the chemistry of the original recipe.
Insight-live shows recipes in tall narrow panels. They open side-by-side right-ward. They remember the type of calculation last requested. So just opening multiple recipes automatically enables comparison.
The test bars will measure fired porosity and shrinkage over a range of temperatures, drying shrinkage, LOI and pugged water content. They follow procedures defined in Insight-live.com.
How to reference a picture from an external website like flickr.com from within a recipe in Insight-live
A short annotated video of how to create an account at insight-live.com
A short annotated video of how to sign-in to a personal account at insight-live.com
How to import the Digitalfire Insight recipe database file (INSIGHTDATA.DB) and the pictures that attach to recipes therein
Using help, your account, renewal and preferences pages, the managers and panels, recipes, materials, entering a recipe, chemistry, downloading desktop Insight.
How to find them, duplicate them and develop them within your account at insight-live.com
If your recipe is on the clipboard, this shows you how to import it into Insight-live and make adjustments after.
Learn how to add a recipe, title it, add lines and change them, set lines to added status, enter notes and pictures and print a mix ticket
An example of how to enter test results from your ceramic testing into recipes in your account at insight-live.com.
How to take a picture using an iPhone, crop and resample it, save it, then upload it to a recipe.
How to import data from desktop Insight, GlazeMaster, Matrix, GlazChem, HyperGlaze, Generic Spreadsheet CSV into your account at insight-live.com.
How to add and override material data and how to do chemistry calculations in your account at insight-live.com.
How to organize your recipes into a worksheet of recipe rows and material columns, save it as a CSV file and import into Insight-Live.com
A tour around the home page. Where to start.
I will show you some secrets of making a base engobe (or slip) apply to leather hard terracotta ware in a thick, perfectly even layer.
I will show you how to glaze a mug with a liner glaze inside and a colored one outside so that they meet in a perfect line at the rim.
To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.
I will show you why people love/hate this material and how I substituted it for Ulexite to make a much easier-to-use glaze that fires just as good or better.
Making 10 gram balls of your glaze and firing them on 2in by 2in tiles is a great way to evaluate their flow, surface and susceptibility to defects.
I will show you why thixotropy is so important. Glazes that you have never been able to suspend or apply evenly will work beautifully.
D. Desktop Insight
Part two of a complete tour. It includes using targets, setting calculation types, entering recipe notes and details, SQLite and a review the menus.
Part one of a complete tour. The anatomy of the recipe window, how to open, edit and save recipes; the materials, oxides and supply oxide dialogs, the MDT.
Learn to how to download a recipe library from your account at Insight-live and open and explore it using desktop Insight
Compare calcium carbonate (whiting) with other sources of CaO (dolomite, wollastonite, frit), learn to understand the chemistry differences between materials and then substitute wollastonite for whiting in a specific recipe.
While comparing a real-world and theoretical feldspar learn to enter, edit, save, normalize recipes and the materials dialog. Glaze chemistry concepts.
Learn to compare a target formula with the chemistry of a feldspar. See why it does not make a good glaze by itself and what materials need to be added to make it into a balanced glaze.
Learn to do difficult formula to batch conversions. Learn mole%, finding frits by chemistry, Na2O sourcing, oxide oversupply, recipe line added status, overriding in the Supply dialog, when to compromise an exact match.
Learn to add a native volcanic ash to the INSIGHT materials database (MDT) and then create a glaze from it maximizing its percentage. Learn to impose an LOI on a material and why this method is better than line blending.
Learn to use a non-unity calculation to convert a formula into a batch recipe using theoretical and real-world materials. Retotal, round-off and make a side-by-side report.
Learn to convert a glossy glaze into a matte by comparing its chemistry with a target matte formula. Alter the chemistry in such a way that the thermal expansion does not rise and it maintains good physical application and suspension properties.
Shows four different ways to add materials to the desktop Insight materials database (MDT)
Do this completely outside of Insight, it knows how to read it at each startup.
There are five ways to do it. Generate your MDT at digitalfire.com, copy and paste XML, type in the formula, enter an analysis as a recipe, handle the MDT as a CSV file in Excel.
Wollastonite is 50:50 CaO:SiO2. So why not just substitute 40 wollastonite for 20 calcium carbonate and 20 silica?
Learn to substitute Nepheline Syenite for Soda Feldspar (and vice versa) using the KNaO checkbox to. You will see the benefit of in-recipe substitution calculation rather than making substitution rules.
Learn the chemistry differences between cone 10 and 6 glazes and how to make a glaze melt at a lower temperature without introducing other problems like crazing.
Learn to reformulate a glaze that is settling in the bucket. Al2O3 and KNaO are sourced by the feldspar, we will source them from kaolin and frits instead.
How to use desktop Insight to substitute wollastonite for calcium carbonate (and vice versa) while maintaining the same SiO2 level. Create substitution rules.
Use Desktop Insight to explore ways of calculating substitutes for Gerstley Borate in the popular Floating Blue cone 6 glaze recipe while maintaining or improving the other raw and fired properties of the glaze.
How to fine-tune the thixotropy of a ceramic engobe for pottery
Tony Hansen takes you through all the steps from opening the box and wedging the clay to taking the fired mug from the kiln.
*Youtube Video, +MP4 Video, #ScreenCast, ^URL
Lesson 3 - Dealing With Crazing
Learn what crazing is, how it is related to glaze chemistry, how INSIGHT calculates thermal expansion and how to substitute high expansion oxides (e.g. Na2O, K2O) with lower expansion ones (e.g. MgO, Li2O, B2O3).
D. Desktop Insight
Click here to watch this at youtube.com or click here to go to our Youtube channel
This video explains what crazing is, how it is related to glaze chemistry, how INSIGHT calculates thermal expansion and how to substitute some of the amounts of high expansion oxides that often cause crazing (e.g. Na2O) with lower expansion ones (e.g. MgO, Li2O, B2O3). The side effects of these substitutions are discussed and the reader is shown how to research the purpose of each of the oxides in a glaze formula. Click here for a page having a link to it.
Lesson 3 Transcript: Dealing With the Root Cause of Crazing
What is crazing, why glazes craze, thermal expansion, stress testing, researching, substituting KNaO, adding more SiO2 and Al2O3 via boron, the unique character of Li2O, adding boron from frits and silica:alumina ratios
Dealing with the root cause of crazing
Welcome to lesson 3. We are going to get to the root of why glazes craze
And Ill talk about thermal expansion, stress testing, researching, substituting KNaO, adding more SiO2 and Al2O3 via boron, the unique character of Li2O, adding boron from frits and silica:alumina ratios.
Thermal expansion information at Digitalfire
As already noted, you can learn more about crazing by clicking the Reference Database link on the digitalfire.com home page, then Articles. There is a section on Thermal Expansion and if you do an on-page search for the word crazing you will find more.
What is crazing?
There are many listings there in the Articles/Glazes section. This one shows the best solution. There is also a very concise one in the Troubleshooting section that explains what it is and how to fix it.
Crazing occurs when a glaze has a higher thermal expansion
than the clay body it is 'glued' to. Most solids expand on heating, and contract on cooling. If a glaze shrinks more than the ware during cooling in the kiln, it becomes 'stretched' and seeks to relieve the stress by forming a network of cracks.
The key to fixing crazing
While there are many band-aid approaches, the key to fixing crazing is the reduction of glaze thermal expansion (or increasing body expansion). INSIGHT can calculate an approximation of glaze expansion as you can see here with the G1214Z recipe. This is a base matte very similar to the one developed in the last lesson.
Boiling water:Ice water test
It is very important to realize that although a piece might not be crazed out of the kiln, the crack pattern can develop over time as it is repeatedly heated and cooled in use. So to test you need to stress the piece thermally to accelerate time. You can do this using the boiling water:ice water test. It is described in the Tests section in the Digitalfire Reference Library.
Adding silica, a poor option
People often suggest a simple silica addition to stop crazing. But alumina mattes need low silica, adding even a few percent can turn them glossy (notice the SiO2 is at the minimum in the target formula to achieve the matte effect). Furthermore, mattes are often not well melted, silica is refractory, it will make this worse. Finally, large amounts of silica are required to dilute the expansion-raising effect of relatively small amounts of oxides like K2O and Na2O.
So we need a better solution or a solution of which it is a part.
Method 1: MgO for CaO
For my first approach to the problem I have opened G1214Z into recipe 1 and copied it to recipe 2 using this button.
I have set both recipes to calculate RO Unity and am going to work on 2 and compare it with 1.
Notice I have the lessons materials database selected as with other lessons.
Analyzing the formula
Take a look at the formula. CaO is very high compared to the limits. What limits?
I have them open from the last lesson, notice the title bar shows which one.
Can I substitute some of the CaO for another fluxing oxide of similar character and lower expansion? Yes.
Magnesia (MgO) also mattes middle fire glazes and has a much lower expansion. I will double-click the CaO oxide line in the Formula List.
Compare MgO/CaO expansions
The Oxides dialog opens.
Notice the expansion number. Now I will click 'MgO' in the list, notice now much lower it is. I will close the Oxides dialog.
Using the detail report to investigate CaO sources
To see which materials are sourcing the CaO I have selected Detail Recipe Calculation from the Report Type menu (in the File menu).
This report shows how INSIGHT calculates the chemistry. INSIGHT has written it to this text file (in the Insight folder in my documents folder) and asked the operating system to display it. In the Preferences dialog you can configure INSIGHT to ask your operating system to display reports in your web browser or system text editor.
This window is narrowed so we can only see the left most portion but notice the CaO column. Clearly, wollastonite is supplying the major portion of the total. Wollastonite is calcium silicate, so why not source MgO from magnesium silicate, namely talc.
Talc as a source of MgO
I have chosen Edit Materials from the Utility menu, entered Talc here and pressed the go button to search and got three hits. I have selected Nytal Talc. Notice this talc is mainly MgO and SiO2 but also has some CaO. Talc also has an LOI (like whiting but much lower) and this can contribute to the formation of suspended micro bubbles in the fired glaze. However this is a matte glaze so they wont be as visible (unless of course they break at the surface).
I have clicked generic talc. Notice it only has MgO and SiO2 and no LOI is specified. Of course such a theoretical material does not exist in nature. The best talcs approach this chemistry better. You might question the selection of talc over a frit to source MgO. But for demonstration purposes using a material like this helps focus on the issues more clearly, we will leave the complexities of incorporating a frit for a later lesson.
Reducing wollastonite for talc
To incorporate some talc I have made sure recipe 2 is selected and changed the amount of wollastonite to 20, that reduces the CaO it contributes to make room for MgO.
I have also checked this box to get the target to weave into the list better, it does not separate K2O and Na2O because they are so similar.
Resultant formula changes
Notice that I have checked the .000 check box above the formula list. Most engineers argue that ceramic calculations are not nearly accurate enough for this, they are right. However some like three decimals to see how rounding was done to two. I will turn this back off.
But first, notice how much the B2O3, Al2O3 and SiO2 have increased as a result of the wollastonite reduction. I am going to add talc one-part-at-a-time to drive these numbers back down.
Driving CaO down with talc
To do this I have clicked a blank line below the recipe and entered Talc in the Lookup blank and updated the line.
Then I will make sure the increment by field contains a 1 and start clicking the Increment arrow and watch the B2O3 and Al2O3 amounts come down (in formula 2).
You might think that the CaO has not dropped much as a result in the loss of the wollastonite, but stay tuned to see what happens.
I kept clicking till the alumina and boron matched formula 1, this happened at 7 talc.
Now the CaO and MgO total what the CaO alone was before. The addition of the MgO drove the CaO down because fluxes are retotaled to one.
The SiO2 was off a bit, (wollastonite and talc don't have the same SiO2 content).
I have clicked the silica line in the recipe and taken it down by one to match them back up.
Assessing the effects of MgO
Take a look at the Calculated Items list. The expansion has dropped from about 7 to 6.6. This is a significant change.
However, before continuing I want to stress that matte glazes have tight limits on their chemistry and do not afford a lot of room to juggle things without losing their surface quality. The addition of MgO might affect stain colors, for example. For now I am going to ignore that.
MgO might also adversely affect melting, so a strong flux might be needed to help. Lithia is really powerful and small amounts are often used for this purpose. I want to demonstrate something about it so I am going to replace all the MgO with it (I would not do this in practice since it would gloss the glaze, but I want to show you something).
Method 2: Trade CaO for Li2O
To change the talc to lithium carbonate I click the talc line in the recipe list and enter enough of the name in the Lookup for INSIGHT to recognize it.
I have also emptied the label so INSIGHT will change it also.
Then I will update the line.
Comparing talc and lithium
Something interesting happens. 7 parts of lithium carbonate supplies 0.24 Li2O to the formula whereas 7 parts Talc supplied only 0.16 MgO? Why is this?
7 grams of lithium carbonate powder sources many more molecules of Li2O because they are alot smaller, remember formulas compare numbers of molecules. Lets look at the weight of Li2O in the Oxides dialog.
Comparing Li2O, CaO, MgO
It is only 29.8, whereas CaO is 56.1 and MgO is 40.3.
Amazingly, lithium carbonate has a higher LOI than whiting or talc. Yet still it supplies many more molecules per weight unit.
You can learn more by clicking the Info button. This page at the Digitalfire Reference Library explains that lithia is a very powerful melter and it can have a disproportionately large reducing effect on thermal expansion. Understanding this and how to fire it is actually the secret behind Corning ware.
Second guessing INSIGHT on Li2O
Notice that even though I added considerable lithium, the calculated thermal expansion is only slightly lower.
This is an example of when we need to second-guess INSIGHT. The relationship between the amount of lithia and its effect on thermal expansion is not linear so it does not 'calculate' as well. From personal experience I would be confident that 5% is enough to fix the crazing problem however it will also affect fired visual character to a greater degree than other fluxes.
To finish I would need to reduce the amount of lithium to match up the boron, alumina and silica.
Method 3: Add B2O3
Lets look at another way to reduce thermal expansion:
Boron is a low expansion and low melting glass, its presence makes room for more very low-expansion (but refractory) alumina and silica (I mentioned silica at the beginning of this lesson). Where do we get more boron?
INSIGHT provides a way to search for materials having a certain oxide.
Using the delete line button
To start again I will copy recipe 1 to recipe 2.
Notice the line label is still there. INSIGHT left it there but I will delete it by clicking the delete line button.
Then I will select the Frit 3124 recipe line for recipe 2 and click the Delete button to zero its amount.
Notice again that it zeroed the line but did not remove the name. This time clicking the button again will not remove it because the other recipe still has some of this.
Now I will select a blank line below the recipe.
Choose a line material using the materials dialog
I am going to determine the material for this line by opening the Materials dialog. I have selected B2O3 in the Having popup and then I have clicked the Frit 3195 line.
Notice this has 23% boron, that is higher than the vast majority of boron frits so this will be a good source of B2O3. Frits are the best source of B2O3, the natural ones have LOI, consistency and solubility problems.
Now I will click the Update Recipe Line button, this will insert this material into the blank line I selected.
Adding B2O3 from a frit
The Materials dialog closed and here it is.
I have already also keyed 36 for the Amount and Updated. I clicked the frit line again since INSIGHT moved the cursor down after the Update button.
How much B2O3?
Notice that the B2O3 is up dramatically, the alumina is also up. That's what I wanted. Extra B2O3 will make the glaze melt lower and increase its ability to tolerate high melting low expansion SiO2; but how much do I add? Another number that INSIGHT calculates is useful. Look at the SiB:Al line in the Calculated Items list.
This is simply the total amount of SiO2 and B2O3 divided by the amount of Al2O3, or a ratio of glass formers to intermediates/stabilizers. If this ratio is maintained the altered glaze is more likely to have the same degree of fired gloss. Testing will be needed to assess comparative melting.
The SiB:Al ratio
Notice what I have done here: I selected the silica line in recipe 2 and clicked the increment button until the SiB:Al ratio numbers matched. It only took 3 parts of added silica.
Notice that the calculated expansion is again reduced considerably.
I can tell you from experience that this amount of added boron will melt more silica and alumina than I have added so there is room to move the expansion lower using this technique.
The best methods all rely on ceramic chemistry!
In most cases, you will find that the glazes most likely to craze are those having significant Na2O and K2O and those lacking Al2O3 and SiO2 (in comparison to limit formulas). Often glazes have both problems. The most effective method of reducing thermal expansion then is obviously reducing KNaO in favor of other fluxes, especially MgO, it has the lowest expansion. Increasing boron to make room for more alumina and silica is also an effective approach.
Think about this: All of the oxides I just talked about come in materials that contribute other oxides also. There is no way you could figure out how to juggle the recipe to effect the changes we have done without ceramic chemistry calculations.
Crazing after a year of use. What is the problem?
A cone 10R grey stoneware mug that has begun to craze on the inside. The greyer coloration around the craze lines indicates that water is soaking into the slightly porous body. This mug has lost the ring it had when it was new (it is only about a year old). It could be refired to be as good as new but would soon return to this condition. The only real solution is to reformulate this glaze to reduce its thermal expansion.
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