Jump to content
MFP

Pyrophyllite ( aka Desert talc)

Recommended Posts

I discovered in my hunt to try to figure out why Mark needed old talc for some of his recipes, that what we used to call desert talc was a substance that was deposited in layers like shale....and that there were two almost identical forms,.....the desert talc and pyrophillte.  The talc we are getting now is literally ground soapstone and is not even the same as desert talc. Apparently if you look at the two under a microscope the difference is readily apparent. I then discovered that it is mined in North Carolina and is markedted as Pyrax by the same company that makes Vansil Wallosanite feldspar.  Ceramic Supply in Pittsburgh carries it. I got ten pounds....and yes....it is beautiful white fluffy talc.  I am sending some to Mark to see if it in fact works in his old recipes. I am also suspicious that I will need it for my old cone 10 recipes as well. So....this post is just an FYI for anyone having problems with the new talc. This is an alternative to try. 

Share this post


Link to post
Share on other sites

Do you have an analysis for the Desert Talc?

From Vanderbilt Minerals their Vansil Pyrax aka Pyrophyllite analysis is this:

Typical Chemical Analysis % (calculated as oxides) Aluminum Oxide  18%, Silicon Dioxide 77%, Iron Oxide 0.5%, Titanium Dioxide 0.2%, Calcium Oxide <0.1%, Magnesium Oxide  <0.1%, Sodium Oxide 0.2%, Potassium Oxide 1%, Ignition Loss (1000oC) 3%  Note the magnesium is virtually nil.

So basically it's 18% alumina plus 77% silica with a few trace minerals.

From Ron Roy's Typical Analysis of Materials all three brands of talc he has analysis for, Natal (no longer available) Amtal (Texas talc but also sold under other names) and Imerys Sierralite talc, fall in the range of 32-55% silica, 0.31 up to 21% alumina (the latter being the high alumina Imerys talc) plus calcium up to 8% and magnesium in amounts of 29.5 up to 34%. There are also some trace minerals. Note the magnesium amounts.

 

 

Share this post


Link to post
Share on other sites

Two different minerals with similar physical properties are talc and pyrophyllite. Their physical properties are nearly identical. Both are very soft: talc is the softest mineral on the Moh's hardness scale at 1, and pyrophyllite is 1 to 2.Both talc and pyrophyllite have perfect cleavage in one direction. This means that these minerals break into thin sheets. As a result, both feel greasy to the touch (which is why talc is used as a lubricant). They are both formed in metamorphic environments as the result of changes in silica-rich dolomite. 

Steatite and soapstone are impure, massive forms of talc that lack the distinctive cleavage mentioned above."   These are the forms that are currently being sold to Laguna.  This is not the same as pyrophyllite.

So maybe talk to the Laguna guy about getting some North Carolina talc. The chemical formula for the pyrophyllite is :

Al2Si4O10(OH)2

Share this post


Link to post
Share on other sites

Yeah but the pyrophyllite lacks magnesium which is why most people use talc, because it is nearly lossless when fired.  You can take a normally gassy glaze that uses dolomite and substitute out part of it for the magnesium, and substitute wollastonite for the calcium part and make a lower expansion and lower LOI glaze.  People don't use it for it's raw physical properties.

Share this post


Link to post
Share on other sites

Maybe in CURRENT recipes....but I am talking about people who are having trouble with OLD recipes because this new talc has those impurities in it.  OLD recipes rely on the OLD talc.  This talc is used for a different purpose.

 

PYROPHYLLITE

PYRAX® B Pyrophyllite is used in rapid-fire ceramic wall tile bodies. It lowers firing temperature; produces low moisture
expansion bodies with good craze resistance; increases thermal shock resistance; and greatly increases firing strength in
vitreous bodies. PYRAX B promotes the development of mullite when substituted for an equivalent amount of feldspar or
quartz.
PYRAX ABB Pyrophyllite is generally used for the same applications as PYRAX B, but it shows wider variations in mineralogy
and particle size distribution.
PYRAX RG refractory grade pyrophyllite is used in refractory mold washes insulating firebrick, metal pouring refractories,
alumina-silica monolithic refractories, ramming mixes, gunning mixes, castable mixes and kiln car refractories.
Al2O3 17% 21% 0. 7% 38% 36%
SiO2 77% 72% 50% 45% 46%
Fe2O3 0.4% 1.9% 0. 2% 1.0% 1.7%
TiO2 0.2% 0.5% < 0. 1% 1.5% 1.4%
CaO <0.1% 0.1% 45% <01% <0.%
MgO <0.1% 0.1% 1.5% 0.2% <0.1%
Na2O 0.2% 0.4% < 0.1% <01% <01%
K2O 1.2% 0.3% < 0.1% 0.5% 0.4%
LOI 3.6% 4.0% 1.6% 14% 14%
For technical questions regarding these products or their use in your application, please contact our R&D center at
MineralLab@vanderbiltminerals.com or click on the Technical Inquiry link on any www.vanderbiltminerals.com website page.

Edited by MFP

Share this post


Link to post
Share on other sites

Talc needs to have magnesium in it or it's not talc. Virtually no magnesium in Pyrophyllite (it's less than 0.1%) therefore it's not talc.

If we could find an analysis of the old talc then you could get close to matching it by adjusting the glaze recipe. Closest I could find was Ron Roy's analysis for Natal (in my first post). Main difference between Natal and what's being sold as Amtalc (also sold under other names) talc is the former has just over double the calcium. Some of the older talcs also contained asbestos.

Share this post


Link to post
Share on other sites

You are missing the point completely. What I found is what used to be called "desert talc" and was called for in old recipes was identical to pyrophyllte and was NOT stearite,,,,aka soapstone....and is why old recipes that call for "desert talc" do not work with stearite because of the magnesium content. I didn't name it that....someone else did. It is not a rock like soapstone and is deposited in layers like shale.  So we will see if it replaces the misnamed desert talc in Mark's recipes....and it is likely to be what I will need in my old recipes. 

Here are the four types of talc.....number three is the pyrophyllite group. What is used now is the stearite/magnesium group. In the past, it was different,.

There are four types of talc deposits:
  • Deriving from magnesium carbonates. ...
  • Deriving from serpentines. ...
  • Deriving from alumino-silicate rocks. ...
  • Deriving from magnesium sedimentary deposits.

The type called desert talc is only 10 percent of all talc produced....most likely because it is SO different from the others.

Edited by MFP
add info

Share this post


Link to post
Share on other sites

The deal with the old talc is only in one of my recipes

Thats a true satin Matt -it does not work with modern talcs

all other glazes I use work with modern talcs (I use Sierralite Talc from Montana via Laguna Clay Co.)

It works with Desert Talc (which was what we all used for over 30 years and I ran out of.

also C-30 talc which was in iuse for another 25 years-I have small amout left as others her have sent some to me.

also works with

NYTAL 100HR

 talc which is also called New York Talc and had asbestos in it and was removed from the market (I still have some)

I'll test the new stuff when I get it.

Most glazes work fine with Pioneer Talc from Texas-its greyish looking as thats what outb there now in most places.

Share this post


Link to post
Share on other sites

Since it has minimal magnesium that may be the difference...and it has more alumina. But....I don't know why it is in the talc category, especially since now magnesium is expected in talc.  But I know 30 years ago..."desert talc" was what was used. 

Share this post


Link to post
Share on other sites

There are far more forms of talc than are used in pottery.....and it is possible that over the years, depending on which mines were operating.....people got sold "talc" of all kinds...not very helpful when trying to replicate old recipes and  vendors have used this generic term for what are distinctly different minerals. The stearite group is what we are using now as "talc". 

Varieties of TalcHide

Beaconite A fibrous variety of talc resembling asbestos.
Chromian Talc A Cr-bearing variety of talc.
Polyphant Stone A greyish-green potstone flecked with whiteand brown. Used since Norman times as an ornamental stone in churches.
Pseudolite Octahedral talc pseudomorphs after spinel.
Steatite A massive variety of talc with a greasy feeling, often used for ornamental carvings.
Zincian Talc Zn-bearing variety from the "Mixed Series" formation, Nežilovo, Macedonia. Associates, i.a., with ferricoronadite.

Relationship of Talc to other SpeciesHide

Other Members of this group:
Ferripyrophyllite Fe3+Si2O5(OH) Mon.
Minnesotaite Fe2+3Si4O10(OH)2 Tric. 1 : P1
Pyrophyllite Al2Si4O10(OH)2 Tric. 1
Willemseite Ni3Si4O10(OH)2 Mon.

Common Associates

Share this post


Link to post
Share on other sites

Finally found one reference with an formula for Desertalc 57, formula is  H2Mg3O12Si4. Hydrogen, magnesium plus oxygen plus silica.  Reference is here.

@MFP, perhaps this Desertalc 57 is not "Desert Talc", but yet another talc, but I did come across Dersertalc several times but not Desert Talc. If Pyrophyllite works in your recipes in place of Desert Talc then wonderful, glad you got it sorted.

Share this post


Link to post
Share on other sites

I've never heard of anything being talc that does not contain magnesium. It's possible that someone in a certain region got ahold of something that had a specific manufacturer's name and used it in some recipes, but isn't necessarily accurate terminology for ceramics. If pyrophyllite works instead of Desert Talc, go for it. In ceramics, talc means magnesium, regardless of the terminology used outside of ceramics.

Share this post


Link to post
Share on other sites

Did you see the above grid that says Pyrophyllite-Talc group?  What I am thinking is that we were sold "desert talc" that was pyrophyllite which is why it is so different in the glazes calling for it==because it doesn't have magnesium. . I am NOT suggesting it is true talc.  I bought this from Ceramic Supply in Pittsburgh.....so someone is using it for something. If it doesn't work for Mark...then it has simply been ruled out. He said something about Texas talc kind of working and that was where pyrophiyllite was being mined before that mine shut down. Before that was the Mojave desert. Now only in North Carolina. I have put hours of research into what "desert talc" was......and discovered it wasn't really talc at all but the industry calls it that. 

So..it is simply an option for Mark to try, If it doesn't work....then some other kind of screwy talc is the reason. IN researching this, I found that the talc is often mixed with other veins of minerals like Wallosanite, Dolomite, and calcium deposits. So....if the "desert talc" he was getting was contaminated with a particular blend of talc, Wallosanite and Dolomite....we will never be able to duplicate it.....and would account for the difference as the amount of magnesium would be much reduced.  I actually talked to the mine in Montana.....and every day there is a different type of soapstone coming out of the mine.  It changes constantly.  And they don't do any assays to determine its composition ongoing.  So literally, the talc can vary from batch to batch. 

Share this post


Link to post
Share on other sites

PS---what I also discovered is that although we are interested in the chemical  properties of talc.....the people mining it are much more interested in its physical properties.  The reason they call pyrophyllite a talc is because their physical properties are very similar. I can tell you in handling this powder that it acts exactly like talc....sticks to your fingers etc. 

Sorry....the digitalfire is not consistent with what I have previously found. I am not going to argue about this any more. I was trying to help Mark out. Since I bought the pyrophillite....he is not out anything to make a 100 gram batch to test it for his one glaze.  I am also talking about chemicals that existed 35 years ago.....and things were much different then.

Share this post


Link to post
Share on other sites
5 minutes ago, MFP said:

PS---what I also discovered is that although we are interested in the chemical  properties of talc.....the people mining it are much more interested in its physical properties.  The reason they call pyrophyllite a talc is because their physical properties are very similar. I can tell you in handling this powder that it acts exactly like talc....sticks to your fingers etc. 

Sorry....the digitalfire is not consistent with what I have previously found. I am not going to argue about this any more. I was trying to help Mark out. Since I bought the pyrophillite....he is not out anything to make a 100 gram batch to test it for his one glaze.  I am also talking about chemicals that existed 35 years ago.....and things were much different then.

Yeah, but desertalc I linked above was made before the 80s and became unavailable.  I have a hard time believing that you are talking about two different products with the same name and from the same period in time

Edited by liambesaw

Share this post


Link to post
Share on other sites

After what I  have discovered about the variability of talc deposits and all the different minerals being sold as talc.....I would not be surprised at all,,,,especially considering that the ceramics community was 1% of their market....and physical properties were what they were after, not chemical properties---which don't seem to concern them at all..  Bottom line....pyrophyllite is considered a talc....not the talc we use in the pottery community....but it is used in other ceramic applications.  And like I said....Mark is not out anything to try it in that glaze recipe. 

I know when I am doing tests of my old glaze recipes, I am going to one of each....because this ugly grey stuff is nothing like what we used to use. I sent what 30 year old talc I had to Mark already, but I only had a couple of pounds left. I had not seen this stearite stuff at that time. I can tell you.....it is not the same as what I used in the past. If pyrophyllite isn't what it was.....we are both screwed in terms of our old recipes. Since Mark had already told me that all the current ones did not work......it was clear that magnesium might be the issue.....and pyrophyllite has hardly any. 

So...if this "talc" doesn't solve his problem with that one glaze....I have no further suggestions--- as it is clear that the chemical properties change from mine to mine and within the same mine on any given day. 

I got a lot of info from the US Geological service. That's how I found the North Carolina mines. 

Share this post


Link to post
Share on other sites

Here are two authoritative sources for the meaning of “talc” and pyrophyllite.  The more recent source is listed first.

TALC

Talc:  A member of the 2:1 phyllosilicates (sheet silicates) with a composition Mg6[Si4O10]2(OH)4; sp. gr. 2.58-2.83; hardness 1 (it has the lowest hardness on the Mohs’s scale of hardness); monoclinic; rare crystals are tabular, often massive; white to green; cleavage perfect {001}; massive talc (soapstone or steatite) can be formed during the low-grade metamorphism of siliceous dolomites; and as a secondary mineral during hydrothermal alteration of ultrabasic igneous rock along shear planes. It is associated with serpentization with serpentine changing to talc and magnetite by addition of CO2.  It is used extensively as a mineral filler.  Reference:  Allaby, Michael.  A Dictionary of Geology and Earth Sciences, 4th ed., OUP.  (2013).

 

Talc: 

1.      An extremely soft, light green or gray monoclinic mineral Mg3Si4O10(OH)2. It has a characteristic soapy feel and a hardness of 1 on the Mohs scale, and it is easily cut with a knife. Talc is a common secondary mineral derived by hydration of magnesium silicates (such as olivine, enstatite, and tremolite) in basic igneous rocks, or by metamorphism of dolomite rocks, and it usually occurs in foliated, granular, or fibrous masses. See also: steatite.

2.      In commercial usage, a talcose rock; a rock consisting of talc, tremolite, chlorite, anthophyllite, and related minerals. It is used as a filler, coating and dusting agent, in ceramics, rubber, plastics, and lubricants. 

Reference: Bates, R. L. and J. A. Jackson. Dictionary of Geological Terms, Third Ed. American Geological Institute. pg. 514. (1984).

 

Pyrophyllite

Pyrophyllite:  Uncommon silicate mineral Al2[Si4O10](OH)2 belonging to the phyllosilicates (sheet silicates) and with properties to muscovite; sp. gr. 2.65-2.90; hardness 1-2; occurs as a secondary product from the hydrothermal alteration of feldspar, and as foliated masses (see FOLIATION) in metamorphic schists.  It has been mined as a substitute for talc.  Reference:  Allaby, Michael.  A Dictionary of Geology and Earth Sciences, 4th ed., OUP.  (2013).

Pyrophyllite:  A white, gray, or brown mineral AlSi2O5(OH).  It resembles talc and occurs in a foliated form or in compact masses in quartz veins, granites, and esp. metamorphic rocks. Reference: Bates, R. L. and J. A. Jackson. Dictionary of Geological Terms, Third Ed. American Geological Institute. pg. 413. (1984).

 

My observations and comment:

Both materials, talc and pyrophyllite are in the same structural network – sheet silicate – but chemically they are different – one has aluminum and no magnesium and the other has magnesium and no aluminum.

The elemental makeup of the two minerals clearly indicates that the minerals are not interchangeable in a recipe of either clay body, glaze, or slip. They may be interchangeable where the material mechanical properties are the key reason for use and the elemental makeup are not important – such as for a dusting powder, filler, or insulating concoction.  In ceramics, the usefulness of a mineral depends on the elemental composition since the ingredients must react with each other to produce a particular  homogenous melt or a particular solid composite where the bonding among the particles is chemical rather than frictional.

LT

sidebar: My experience has been that the studio pottery literature has often ignored the scientific definitions of many of terms and created their own jargon (but then, Ik ben een ingenieur!).  The same goes for material brand names.  I suspect that the term Desert Talc was a tradename (or local jargon) for a powdered rock from a certain desert mine that some potter used as his “unique” ingredient for making his pots seem different from all the others.  :D

Share this post


Link to post
Share on other sites

This was my point.....that pyrophyllite is marketed as talc because of its physical properties, not its chemical properties. Layered talc often was in alternating layers of talc and pyrophyllite. And I agree.....that the name was not in any way related to the contents. What I highly suspect was that the talc we got was from a layered deposit contaminated with Dolomite, pyrophyllite, and Wallosanite.  However, since my friend has tried all the current magnesium containing talcs.....it didn't hurt to give pyrophyllite a test---just in case that is the "talc" we got 35 years ago--- since these marketers only care about the physical properties in most cases  (for example.....one mineral they call "talc" is almost entirely asbestos).. When you look at pyrophyllite and touch it, you could not tell the difference from true talc. 

Share this post


Link to post
Share on other sites
2 hours ago, Magnolia Mud Research said:

one has aluminum and no magnesium and the other has magnesium and no aluminum.

@Magnolia Mud Research,  Imerys Sierralite talc has 21% alumina. (analysis in the Ron Roy link in my first post or here from Digitalfire). I know this isn't the norm for talc analysis, but just adding to complete this discussion.

Just as an aside, for anyone considering the benefits of glaze calc software, this is another case in point about how it really useful it can be. Obviously a recipe with a fair amount of talc in it is going to turn out very differently using Sierralite talc vs one of the more mainstream ones like Pioneer/Texas talc. Adjustments can be made to the recipe to reflect the differing chemistries. (or to replicate the analysis of the Desertalc that Liam posted)

 

Edited by Min

Share this post


Link to post
Share on other sites

Yes....although "talc" is a homogeneous term in pottery meaning "stearite", it is a heterogeneous term in mining....referring to at least 8 different minerals depending on the application.  Pottery was even less of a market share 30 years ago than it is now.....there's no telling what we got when we were using "talc".  I know we had to abandon at least two glazes because one of the materials that showed up were no longer the same....and we couldn't get what we had previously.  I am sure the university was using soapstone....but other people were using whatever people said "talc" was....glaze theory and glaze calculation was not the norm in the pottery community.....you had recipes and you followed them....if a material changed....you just did not have that glaze any more. Thankfully, times have changed. 

Share this post


Link to post
Share on other sites

Interesting research @MFP!

Interesting discussion in that I just sent back to a Major supplier titanium dioxide which was ordered as rutile. An interesting discussion to say the least as they told me it was their standard practice to send in place of rutile because it was the exact same thing.  

I think I sent back their limestone substitution for dolomite as well because they failed to send me the chem analysis showing me it was reasonably the same. Same supplier kept sending Gillespie for gerstley without informing as well and just stating it’s identical.

End result: stopped ordering from them.

Edited by Bill Kielb

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.