BobMagnuson

I was able to download, extract and use ACerS-NIST PED Demo from: https://www.nist.gov/srd/nist-standard-reference-database-31 It's a fairly large file, so it does take a while to download.

On my system, if I make changes to the spreadsheet and then save it with a new name, the next time I call it up Excel comes up with a Security Warning and I click "Enable Content", then "Yes" to make the file a trusted document. This needs to be done whenever the file name is changed.

The tile on the left is Marcia's Matte. On the right is the same glaze with 20% silica added. It's hard to see in a photo, but the silica made the glaze less matte, but not glossy. I think the matte in Marcia's Matte is created from both excess Al2O3 and MgO. Adding silica changes the SiO2:Al2O3 ratio, but doesn't necessarily affect what MgO is doing. I will agree that I have used the definitions of opacifier and matting agent too loosely, but they are often the same thing. Creating a matte surface automatically creates some opacity. The reverse, however is not true. From https://digitalfire.com/oxide/mgo: "At lower temperatures the matting mechanism of MgO is that it simply stiffens and opacifies the glaze due to its refractory nature."

I did the Marcia's Matte demo without any recipe modifications. If MgO is just a matting agent, I thought, how can adding silica reverse that effect? I don't know, but it did. One possible explanation is, since the effect was not dramatic, the extra silica simply reduced the percentage of talc, and hence MgO, in the recipe.

I decided to run a couple experiments to test some of the recipes discussed here earlier Both were fired to cone 7. I work at a community studio (Duluth Art Institute) and cone 7 was the next firing...I didn't want to wait for the next cone 6. Close enough. First, I made up a batch of Min's "Base Test." (See Oct 1.) Then I took the same recipe and left out the talc but replaced the amount of silica that the talc would have provided. In other words, I just took out the MgO. My test tiles were Continental Clay "B Clay", bisqued to cone 08 and scribbled with a black underglaze pencil. The difference was not dramatic, but I would say the "Base", with MgO, is more "fluxed" than the modification, without MgO. (Notice how the underglaze ran more when MgO was present.) Then, I made up a batch of Bill Kielb's "Marcia's Matte", (See Oct 2) and added 10 and 20% of SiO2. Once again, the results were not dramatic, but the added silica did indeed make the glaze glossier, though still somewhat matte. Notice how the scribbling on the original is a bit less black than the other two. My conclusion is...yes, MgO can act as a flux in midrange glazes. I have to admit that I haven't worked with midrange glazes for quite a while. I've been working at cone 8 to 10 for the past couple of decades. And I rarely use borates for high fire. It just seems like cheating! My original thinking was this: if it's so easy to make a high fire magnesium matte, how can MgO be a flux at a lower temperature? Maybe borates are the key. Maybe. Where to go from here? Surely there's no simple answer.

I was finally able to lay my hands on a copy of Hamer and Hamer "The Potters Dictionary...", and read the section on Magnesium Oxide. The amount of information they present is really quite impressive. I don't want to say they're wrong about MgO being an auxiliary flux producing "late fluidity" between 1190 and 1230 C (roughly cone 6 to 8.) I'm just looking for examples demonstrating that statement. If we can find examples where MgO is clearly an active flux, perhaps we can begin to understand how it works. My thinking goes like this: A flux, in ceramics, is something that lowers the melting point of silica. All of the other R2O and RO oxides (Li2O, K2O, Na2O, CaO, ZnO, SrO, and BaO) do this by forming alumino-silicate eutectics melting in our typical temperature range. The MgO alumino-silicate eutectic point, on the other hand, is about 1350 C, or around cone 13 or 14. There is a magnesium borate "eutectic" that melts much lower, but it doesn't have anything to do with silica. And in that binary system, I would say that boron is the flux and magnesium is the refractory. It will melt and become an important part of the glaze, but it doesn't necessarily help melt the silica. Even if MgO CAN be an active flux at cone 6 to 8, it doesn't always. I would bet most of us have used glazes in this temperature range where MgO is an opacifier. So if Hamer and Hamer are correct, there must be specific conditions for it to become a flux. There are possible explanations for this, including: there could be a simple MgO eutectic mixture that we haven't discovered yet. Or there could be a complex eutectic mixture formed when MgO dissolves in the primary R2O-RO-alumino-silicate melt. (ZnO, for example, requires CaO to form its low melting eutectic.) At this point, we just don't know. So, I would like to find glaze recipes where MgO is believed to be an active flux. Then, if MgO, and only MgO, is removed from the recipe, the glaze should be less mature, or at least less fluid. The more important question here, in my opinion, is that if MgO is not acting as a flux, becoming part of the clear glass part of a glaze, does it belong in the RO flux column of our glaze calculators? If we take it out of the RO column, the R2O in the R2O:RO ratio is higher than we thought and perhaps the glaze is less durable than expected.

I think this is an excellent recipe for the test. My guess is that the modified glaze will be somewhat more fluid, because MgO generally increases the viscosity of the melt. A negative effect of removing the MgO will be that the modified version will be more likely to craze. Thanks for bringing up magnesium borate! When I added borate eutectics to EuCal I didn't find magnesium at all. That's on me. It should be added, and it does explain how a fair amount of magnesium can exist in a transparent cone 6 glaze. But there's no silica in magnesium borate, so it still doesn't explain how it might be acting as a flux.

Min. Thanks so much for your questions! The whole reason I presented my EuCal spreadsheet in the first place was to start a discussion on this way of looking at glaze calculation. I would never say that Linda Bloomfield is incorrect, just that I have not been able to find the data to back it up. That's not to say it isn't out there, just that I haven't found it. There are zillions of phase equilibrium studies out there, many behind paywalls beyond my reach. As to MgO-CaO-Al2O3-SiO2 eutectics, the only ones I have been able to find, so far, have liquidus temperatures above 1350C. I'll keep looking though. And I'm still looking for a glaze where MgO is an active flux.

I don't have a maximum MgO for you, but I would start in the 0.2 to 0.4 range.

I think of all glazes as being made up of two parts: the clear glass base and the modifiers. The clear glass base is made up of a primary eutectics (usually those of K2O, Na2O and CaO). The modifiers are any oxides that don't fit within the eutectics. They might be dissolved in the base, or they might be suspended in it. Some might be dissolved in the base when it's melted, then crystalize out during cooling. The eutectics themselves never crystalize...they form the clear glass phase. The modifiers are usually colorants and opacifiers. Some oxides, like CaO, SrO, or BaO are usually fluxes that form eutectics, but if either SiO2 or Al2O3 is short, they will simply be opacifiers. I previously used an example of a white glaze that was opacified by an excess of CaO. It could be turned into a clear glaze by adding EPK to fill out the CaO-Al2O3-SiO2 eutectic. A ceramic flux is an oxide that lowers the melting point of silica, usually by forming an alumino-silicate eutectic. Since the melting point of the MgO-Al2O3-SiO2 eutectic is somewhere around cone 13, I'm not at all sure that magnesium ever acts as a flux, regardless of whether it comes from dolomite, talc, or a frit. It's certainly an important glaze ingredient for its other properties. It's just not a flux. But I could be wrong about this. If you disagree, show me a glaze that would be less mature if you remove the MgO. For some background and my method of calculation, see Techno File: Using Eutectics, February 2018 issue of Ceramics Monthly.

I may be a lone wolf on this idea, but, I think the magnesium alumino-silicate eutectic is unlikely to form at cone 6, and therefore is unlikely to be part of the glass phase of the glaze. So does MgO belong in the flux column? Maybe not. If you take it out, the R2O/RO ratio becomes 0.38/0.62, which would indicate a much less durable glaze. The magnesium could be just an opacifier in this case.

Zinc can be an opacifier when the calcium is low. This recipe has loads of zinc and very little calcium. It's also shy on alumina. In order to reach the zinc eutectic point and prevent it from opacifying, both CaO and Al2O3 will need to be increased quite a bit. I don't have much personal experience with low temp glazes, but I would start by doubling the EPK and adding whiting in 10% increments.

Sorry I'm late to this conversation. I see 3 problems with this glaze. First, the clay content is too high, which can cause shrinkage and separation before firing. It might not be noticeable before firing , but can show up as shivering out of the kiln. Second, the coefficient of expansion is quite low, which could be an issue with some clay bodies. And finally, it looks like it might be applied too thick. The first 2 issues could be resolved by reducing the kaolin by 25% or so. (Run a few tests in that neighborhood.) Finally, if you're dipping your glaze, keep the specific gravity below 1.5. 1.45 works for me with a 5 second dip. If the problem is the clay content, another solution would be to replace some (half?) of the kaolin with calcined kaolin. That way you can keep the chemistry the same while reducing the pre-fire shrinkage.

I used a Wagner spray gun may years ago. It worked very well for a while, but died young. Airless spray guns have the advantage that, without pumping all that air, the glaze arrives on the pot wetter than it does with standard compressed air sprayers. The big disadvantage, as Dick White said, is the abrasiveness of glaze mixtures. Remember that "airless" sprayers need to run the liquid through a pump, so it's not just the tip that wears out. Mine quit quite suddenly and it wasn't merely nearly dead...it was completely and sincerely dead!

Instead of figuring out where the clay "matures", why not just use it for terracotta? If it's plastic enough make pots, handmade or thrown, firing to around cone 04 often makes a beautiful clay body. It won't be vitrified, but could be great for flower pots or sculpture. Otherwise, a 50/50 mixture with wood ash will probably make a decent stoneware glaze.

Scale from water wells will not have any significant R2O. CaO is a flux but CaO-SiO2 alone won't melt until well over 2000 C.

Are you sure it's silica scale? If it's lime scale, CaCO3, heating it up will convert it to CaO, or quicklime. That's easy enough to test, because quicklime will get very hot when added to water. I'm not familiar with geothermal wells, but scale in water wells is usually mostly CaCO3 with some some MgCO3 and a little SiO2.

A demo version of PHASE 5.0 is still available.

The most authoritative resource is probably ACerS-NIST Phase Equilibria Diagrams. The current version is called PHASE 5.0, I think. The big problem is that it is WAY expensive...well over $1000 for a single user. I have never had access to the full version. It's possible some university libraries might have it. In the past, they offered a demo version that I found quite helpful. I don't know if that is still available. As you can see, tracking down reliable information on ceramic eutectics is not at all easy. Good luck!

I can't seem to post an Excel file to this forum myself, so I'll send a copy by email to Min. I noticed that C.Banks is using Open Office. It would be interesting to hear what other folks are using to open EuCal. I only have Microsoft Excel myself.

I do have an update to EuCal 1.9. Dick White was kind enough to update the % Analysis tab. Lots of the data in mine is quite old - because lots of my ingredients are quite old! Anyway, I have version 1.91. If it's Okay with Dick and Min, I'll be happy to share it. It's also fully unlocked.

The evidence suggests that while ZnO is easily reduced to Zn and volatilized under reduction, once it enters the melt the loss doesn't happen, or is at least minimized. I think ZnO can enter the melt either by forming an alumino-silicate eutectic with calcium or simply by dissolving in the liquid glass. Some zinc crystal glazes have little or no calcium or alumina. In that case, EuCal isn't helpful, because the simple binary R2O:SiO2 eutectics are not included in the calculations. If calcium and alumina are present, zinc needs to be "excess to the eutectics" in order to leave enough available to form zinc silicate crystals as the melt cools.

So there are indeed reduction glazes that start off with ZnO in the recipe. But Robins Clear looks like it would work fine at cone 10 without the zinc, so if some is lost due to reduction firing it would be hard to tell. Now, if there are reduction fire zinc crystal glazes, that's a different story. If zinc crystal glazes can survive reduction fire, then zinc loss due to volatilization probably isn't very important.

You are correct in that, under reducing condition, zinc oxide converts to zinc metal and will volatilize around 900C. This could cause glaze immaturity due to loss of flux, or bubbles and pinholes. If reduction is started later in the fire - after the glaze has melted, there may be some protection from the volatilization, but I don't know that for sure. Does anyone out there have a successful reduction glaze recipe that includes zinc?

Hello everyone. I'm the guy who put together EuCal. This Forum is new to me, so I have some catching up to do, but I'll do my best to help out where I can. No calculator can predict everything. Testing is always still needed. When using EuCal, you will find that most glazes will tolerate SOME excess SiO2 and Al2O3 before they start getting opaque. The recipe posted recently could probably be made slightly more transparent by reducing the EPK and Silica a little bit.