Talc as lowfire flux

[ATauer]

Just for those of you who haven’t had the benefit of seeing Matt Katz lecture in his CMW Intro to Clay class were he rips apart all notions that talc acts as a flux in clay at lowfire temperatures, particularly the 06-04 range using a huge amount of data he has amassed through a bunch of experiments. There isn’t that much information on this out there that has been published though, and there are still a lot of clay suppliers who are making lowfire clay with up to 50% of it made of talc. I know there are some ideas it helps with glaze fit and crazing but he also showed us there was no data to support that in lowfire claybodies. Anyway, I came across this little experiment Tony Hansen did on Digitalfire which also includes some interesting visual data about some other materials as well! Matt’s suggestion to us was of course to fire to cone 10, because that is what he wants everyone to do always anyway, lol, but basically urged us if we were to be buying lowfire clays (including reds/terra cottas, talc is put in them a lot as well) to ask the suppliers for a data sheet on the materials in the clay to make sure there is no talc in it, and if the supplier won’t give that to you its time to change suppliers. At the very least you are entitled to the MSDS sheet on the clay and talc is a material they would have to list-just check the date and make sure it isn’t 20 years old which I’ve often found & usually they’d had recipe changes in that time.

 

And no, I am not anti-talc, I quite love talc at higher temperatures in glazes! I’m quite worried about our future with talc, yes I know there is Cim-talc but I’ve been told it performs terribly in glazes by someone who should know, and it seems like any time a mine becomes available it is lost to us 6 months later.  Our materials, as this linked blog post shows, are just so important to us, and without consistency….

https://digitalfire.com/picture/1115

[Mark C.]

Talc bodies have had issues for decades and really where used in low fire clay applications to some extent

This is a common story for low fire folks for  many decades 

As to cim-talc that manufacture mines a few different types and you can look them up.

None of my bodies use talc and the only talc I ever use (have 4  kinds) is for a few high fire glazes in my mix of 15 glazes I use.

When I speak of talc its for glazes not clay bodies

 

[PeterH]

9 minutes ago, ATauer said:

... there are still a lot of clay suppliers who are making lowfire clay with up to 50% of it made of talc. I know there are some ideas it helps with glaze fit and crazing but he also showed us there was no data to support that in lowfire claybodies

Was Max Katz claiming that the talc doesn't increase the thermal expansion (and hence improve glaze fit), if so I'm confused.

In https://digitalfire.com/article/low+fire+white+talc+casting+body+recipe

 

[ATauer]

1 hour ago, PeterH said:

Was Max Katz claiming that the talc doesn't increase the thermal expansion (and hence improve glaze fit), if so I'm confused.

In https://digitalfire.com/article/low+fire+white+talc+casting+body+recipe

 

I would need to go back and find that lecture and watch it again, I paid attention a lot to the rant about talc as a lowfire flux because it was a real hopped up rant which is always entertaining  but since I have no interest in lowfire bodies I was not as interested in the rest of that part, mostly I remember seeing some charts and I think him saying that the other so-called benefits of using it in lowfire bodies didn’t really have the evidence to back up those claims and I thought I saw some proprietary data on it. But I will have to go back and rewatch to make sure about that part. I think the point was like we see in the Digitalfire blog photo I linked to, talc is inert at those temperatures, if it isn’t melting then it can’t be affecting the thermal expansion, because it’s oxides are not active. They are just a powder that doesn’t melt, and don’t interact with the rest of the materials. That’s what I’m remembering as the reasoning. That it of course does very much act to *lower* thermal expansion at cone 6 and cone 10 because of the effects of Mg. That it wouldn’t have any effect either way in a lowfire glaze, either, same reasoning as before, its not melting and interacting with the rest of the glaze materials. As a thought experiment, based off of the photo proving that talc does not melt *at all* at lowfire temps, something I’ve seen before too, pictures of it not melting, then the material doesn’t interact with the other materials during the firing, which it would need to do to increase expansion. I would think that it would just keep expansion the same as if it wasn’t in the clay, as it isn’t lowering or raising it. Tony mentions that all the minerals are just sintered and mostly just remain the same. Which to me would indicate that the Mg wouldn’t do anything to the thermal expansion. I know he shows a graph too, so I’ve got fighting graphs to contend with, but using dilatometers is notoriously tricky and not easy to get accurate results. I would need like a bunch of trials, not just one time that he did this on one clay, but 20 or 30 repetitions and also several versions using different clays with talc in them, (which Matt showed us talc in a variety of clays all not fluxing at all at that temp), that’s what I would do in a clinical trial basically, in order to get the best information, including a control low-fire clay with the exact same ingredients as the first one but without the talc to really evaluate it. I never trust one time dilatometer results. 

[Min]

Low fire "talc bodies", that is to say bodies that employ talc as a significant percentage of their makeup like the typical 50:50 talc to ball clay, or in that ballpark, have never been known as strong claybodies as the talc bonds the clay together by sintering rather than fluxing. Given that talc doesn't give up it's chemically bound water until around 1650F, this means that for low fire glazes fluxed with early melters or even boron (as all lead free glazes are) the glaze will be releasing bubbles of gas well after the glaze has entered the melted phase. This can cause issues with glaze transparency in addition to blisters. This info isn't new.

6 hours ago, ATauer said:

Our materials, as this linked blog post shows, are just so important to us, and without consistency….

There has never been consistency with the materials we use. If you find a material you can't work without then stock up on it because sooner or later it will change. 

[PeterH]

Suggesting/pointing-out that the talc wouldn't be acting as a flux is one thing. Suggesting that this means that a 50% talc body cannot have a high thermal expansion seems -- on Hansen's evidence -- just plain wrong. If it does have a high thermal expansion then claims that this cannot assist glaze fit seem misguided.

Of course, people may have been have been using erroneous arguments to explain why the thermal expansion was high. But disproving those arguments doesn't influence whether or not it actually is high.

[Min]

As the old expression goes the proof of the pudding is in the eating. Perhaps we need a ceramics version of that expression. Hypothetical argument / information versus real world experience. Lowfire talc bodies have been extremely popular because they are so easy to fit craze free glazes on. 

[ATauer]

On 7/31/2022 at 5:36 AM, PeterH said:

Suggesting/pointing-out that the talc wouldn't be acting as a flux is one thing. Suggesting that this means that a 50% talc body cannot have a high thermal expansion seems -- on Hansen's evidence -- just plain wrong. If it does have a high thermal expansion then claims that this cannot assist glaze fit seem misguided.

Of course, people may have been have been using erroneous arguments to explain why the thermal expansion was high. But disproving those arguments doesn't influence whether or not it actually is high.

Hansen doesn’t really have any evidence, that is what I pointed out, he has one data point. I have seen lots of data that I’m not sure if it is proprietary but it had a lot more actual evidence and appropriate scientific method where the control was the same thermal expansion as the clays without talc. I already posted what would need to be done with Tony Hansen’s anecdotal evidence to determine using a dilatometer if the talc actually increases the thermal expansion. His own explanation of what happens in lowfire bodies in fact argues against talc increasing thermal expansion. 

Talc bodies may have always been short and hard to work with what I’m saying is that people are complaining in large numbers throughout the country to the for Flux Sake Podcast and other people that the talc currently being used is much, much worse, to the point they can’t work with the bodies anymore at all. And there has been a clear, rapid change in school and park and small art centers that give lessons to converting to cone 6 instead of lowfire anymore (one of my former teachers did it just as I left her classes, still makes me mad she didn’t do it earlier) because the clays are so awful they can’t teach with them. I’m also seeing in a ton of Facebook forums hobbiests and even professionals asking for advice about cone 6 bodies and glazes as they have just converted because they couldn’t find a lowfire body they could work with anymore, many having been doing lowfire work for years and being very committed to traditional majolica or specific glaze colors that are harder to get at higher temps and doing it because they feel they have no choice. That is a lot different that just the status quo. 

[ATauer]

On 7/31/2022 at 9:43 AM, Min said:

As the old expression goes the proof of the pudding is in the eating. Perhaps we need a ceramics version of that expression. Hypothetical argument / information versus real world experience. Lowfire talc bodies have been extremely popular because they are so easy to fit craze free glazes on. 

See my other post, people are abandoning lowfire bodies in droves because they are completely unusable, that is a lot different than just consistency issues we’ve always had to deal with. From what I hear, lowfire talc bodies are no longer extremely popular and they are also not fitting glazes so easy anymore. It is also not hypothetical- I’ve seen the data, I just can’t share it because it is in a class where everything is proprietary and I can’t share slides otherwise I would show you the data, that is robust and done using the scientific method appropriately. There have been plenty of lowfire bodies without any talc over the centuries that fit glazes fine, lots of terra cottas being used currently and for decades without any talc that also fit glazes fine, lots of wild clay bodies that are terra cottas that do well with glazes, it is very possible there are other reasons and other ingredients. Including the increasing improvement in commercial glazes, how they make them and what they put in them, that especially at low fire make them fit much better than they used to a wide amount of different claybodies, in fact some that fit pretty much every lowfire clay and often if you try even cone 6 and cone 10. 

Consistency has been an issue usually over the course of years for most materials, most mines take quite some time of producing the same material before that is used up and they go deeper and the chemistry changes a little which usually just requires some mild adjustment for the majority of things. There are some exceptions, like rutile, where they always say if you find some you like buy 50 or 100 lb to last you the rest of your career because it changes much more often in the mines and there are many different mines that are used and suppliers change them frequently for some reason without telling you. And over a number of years feldspar mines have been mined out or closed, so *sometimes* changes have to be made for feldspars, a lot kinda close together ran out but before that the feldspar changes had been usually with quite a few years in between- for me  they don’t (not until the Canadian neph sye runs out) because I have a thing about not having a lot of feldspars taking up space, I alter almost every single glaze so I can use neph sye using glaze calc (amazing the number of glazes that has improved significantly)  except the extremely rare times that it has has affected color or COE, but usually when I alter the glazes to use neph sye the calculated COE goes down to my surprise, if it needs Cornwall stone I have found I can’t glaze calc my way out of that but I have switched from using the stuff I can get at my supplier which I don’t know how reliable it is and do the home made recipe for consistency-plus it is all stuff I have on hand anyway. I try and limit a lot of my ingredients- I refuse to use wollastonite as it is a PIA and I much prefer whiting & just adding more silica. I try and alter things so I can use dolomite instead of talc, especially now. It is possible to not have much in the way of consistency problems if you know the materials that are problems- talc, Cornwall stone, feldspars, rutile, some metal oxides, etc, and find ways around them or buy big bags of certain problematic things and when you get to the point you need to resupply test test test. 

Over all consistency is not as much of an issue the same way talc has been an issue over the last few years,  where month to month there are problems and changes. It doesn’t have to be like this in North America either, there are tons of talc mines in other countries, just no company is willing to ship over here. Yes it would cost more and that’s a bummer, but there are huge talc mines in lots of countries with pretty consistent chemistries and enough to last decades. I don’t understand why no companies will import them. Other countries do not have talc issues, they often have plenty of other sourcing and consistency issues but not talc. I’m trying to think of what I’m going to miss when I move to France, where talc is not an issue and I can get most of what I already use or they have consistent supplies of almost the same chemistries as what I have been using so my glazes shouldn’t change too much. I can even get mason stains, which I prefer, although I’d probably adjust to what stains are cheaper. From my conversations with French ceramic artists and my friends in nearby European countries who are fellow glaze nerds I should have a lot less problems getting things and have good consistency, and access to a ton of things I don’t have access to in the US, some things in particular that should improve my work considerably and a number of things that will allow me to do things I can’t do now.  And while I don’t use it much, just in certain glazes, no talc issues!

[Babs]

Basic research practice i passed on to my students:

Without authentication, not worth reading. Would you post photos of your tests and notes.

These forums are full of folk who willingly share knowledge acquired by decades of observation , learning and good practice.

If you diss peple like Tony Hansen, and this is not the place, replace his findings with authenticated info.

Apologies if this is humour.