PinHoling in Ti glazes

[Louis Katz]

Hello Clayers
I am looking for information on why TiO2 .... Rutile causes pinholing in glazes.
Anyone have any theories?
Thanks
Louis Katz
http://LouisKatz.net

[nis...@gmail.com]

I can't say for sure why you're having that issue. One thing to
consider though, is that Rutile is not purely Titanium Dioxide. Maybe
the fact that it increases the amount of silica (slightly) in the
glaze raises the melting point a little? I'd try two things to see if
it helps. One, would be to add a small percentage of flux (like
Custer) to see if that helps. I'd also try substituting straight
Titanium Dioxide to see if it makes a difference. That could rule out
(or confirm) titanium itself as the culprit.

Rutile:

0.81 % SiO2
0.62 % Fe2O3
96.76 % TiO2
0.91 % ZrO2
0.70 % V2O5
0.20 % Cr2O3

I hope this helps!

Denise Joyal

[Rick Mahaffey]

Sone thought on Ti O2 while sitting home waiting for the ice to melt around these parts.

Titanium Dioxide is quite refractory melting at nearly 3000 degrees F. It helps promote crystal formation in glazes. Sometimes pinholes are formed because Ti O2 (or Rutile to a lesser extent (in reduction)) can stiffen the viscosity of the glaze melt not allowing pinholes to heal once they form.

F Carlton Ball used to suggest 1 to 3% Lithium in cases of pin holed glazes because Lithium promotes vertical mixing of the glaze melt helping to get any gasses out of the glaze. It is also a powerful flux due to it's low atomic weight so pound for pound you get more atoms of lithium that you do with other fluxes making it more powerful for a given weight of material.

YMMV, (Your Melt May Vary)

Rick

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[Louis Katz]

The lithium suggestion is what I am hoping to come up with, but I really want a cause first. My experience is that some glazes become very fluid with additions of Rutile or TiO2 , that they seem to have low surface tensions (they spread at the edges), but still pinhole. I am wondering if dissolving TiO2 decreases the solubility of a gas in the melt, or if it by some other means liberates something as a gas. I think that there is something happening that is less simple than run of the mill pinholes.
Anyhow Thanks all.
Louis
plates on the agenda.

[David McDonald]

Bingo Louis. I've used rutile glazes for over 30 years, and this is a reoccurring problem. My understanding has always been that rutile greatly increases the volatility of glazes, causing boiling and gas creation/release. The bubbles in the glaze do not always easily smooth over and heal. But this problem aside, I love what can be achieved with rutile, and continue to use it. 

Try this; a 75% unwashed ash, and 25% rutile mixture sprayed or brushed lightly over a saturated iron red or tenmoku black glaze. Streaking, running, golden brown treasures are the result. And crystallization in cooling is another common added attraction.

David

[Neon-Cat]

Pinholing in rutile glazes and/or with rutile washes is due to the evolution of millions of wee oxygen bubbles in a rare Titanium(IV) → Titanium(III) redox reaction at the interface of the rutile particles and glaze matrix in the temperature range 1200 - 1500 +/- 150°C. One could adjust firing schedules or reduce the surface tension and/or viscosity of the glaze. In a school setting I might opt to add a fining agent to the glaze or rutile wash to help with bubble removal. Antimony oxide (0.4 – 0.6 wt%) would be a good choice and is carried by pottery suppliers.   Rutile contains up to 15% impurities. Because of this, its melting point will be about 40°C less than that of chemical TiO2. The impurities act as fluxes.   Besides the common leucoxene, rutile, and ilmenite impurities, rutile suppliers from around the world report these other compounds and elements: ZrO2, Fe2O3, Al2O3, SiO2, V2O5, Nb2O5, Cr3O3, P2O5, U, Th, Ta, Sn, S, and Al2O3.SiO2 (as sillimanite).   Now I would like to know why college professors don't at least acknowedge inquiries about the possibility of attending their schools. A moot point now, but still the cause of lingering un-groovy feelings.   Marian Gooding Neon-Cat Ceramics www.neon-cat.com  http://www.flickr.com/photos/neon-cat/   --- On Sat, 1/21/12, Louis Katz <loui...@yahoo.com> wrote:

[Bill Merrill]

 

Marian said  “Now I would like to know why college professors don't at least acknowedge inquiries about the possibility of attending their schools. A moot point now, but still the cause of lingering un-groovy feelings” 

 

 I have taught at the college level since 1966.  We didn’t have the Internet and email then, so we would answer students  by mail.  If I could I would call the students and find out if my school was the right one for them, or I would recommend a school that was better for the student in the long run.  Professors for the most part do answer their student inquires for the most part and I do not understand why.    I personally do not know any college art professors that would not answer an inquiry of a student about their school.  That’s my experience, but then I can only speak from my own experience.  I am sure that there are students that were not answered in the past, but that is not the fault of a school,  it is laziness on the part of the person receiving a request from a student.  I have to remember to go to the mail room to get my regular mail, which is something that is becoming antiquated.

 

Students are what we are in the teaching profession for, if not someone is in the wrong business.

 

There are statements about the pin holing caused by rutile.  I have in 50 years of using rutile never had that problem.  I wonder what I am doing wrong?  Ha Ha.  Regular rutile is somewhat of a bland color  in the raw state.  Many years ago you could buy light toned rutile.  It was a beautiful orange color in the raw state but was very similar to the regular rutile after reduction firing.  I liked to do brush work using colemanite/rutile over glazes.  When the light toned rutile was discontinued, I found that if I calcined regular rutile to 1100 degrees F in an electric kiln, it would come out of the kiln a beautiful orange color. I just liked the color to work with….

 

Bill Merrill

--

[Neon-Cat]

You're a good man, Bill. I have learned from you and your work. And have greatly appreciated your encouragement in my new career.   Boron is an excellent choice for rutile washes or glazes. Here in my area I have never seen pinholing from rutile washes or glazes. I had to go find photo examples on the net.   Redox reactions in glazes are very interesting. They're just a type of internal oxidation and reduction that goes on all the time in things we do. Here is the notation for what we are talking about:   4Ti4+ (melt) + 2O2- (melt) ---> 4Ti3+ (melt) + O2 (vapor)   The Ti(III) ion is violet and Ti(IV) ion is colorless. Ti(III) or Ti(IV) are just another way of noting Ti3+ or Ti4+. Anyway, silicate glasses doped with Ti3+ (tetrahedral coordination) are purple. Iron with rutile keeps the Ti ion in its highest oxidation state, Ti(IV). No bubbles.   Titanium acts as a glass former at low concentrations. For concentrations below about 10 wt% TiO2, titanium substitutes for Si4+ as Ti4+ in the silica network.   More on pinholing - another secondary cause: hydrogen pinholing. In fuel-fired kilns where water is always being generated during combustion we can create iron titanium hydrides through reactions of hydrogen with iron-titanium compounds (FeTi, Fe2Ti, FeTi2) above 1000°C (1832°F). These can decompose later to Ti and FeTi below 1000°C. When they decompose they release their hydrogen causing hydrogen pinholing if conditions in the glaze are favorable.   Wonder of wonders, water, as dissolved molecular H2O or as hydroxyls, is considered by some to be more active in glazes and glasses than oxides. Interesting.   There are a few more reasons gases may be generated when working with rutile, but oxygen and hydrogen generation are top of my list for us in our temperature range.   I have rutile light, rutile dark, and milled rutile. I use them or chemical titanium dioxide in my low fire work. Very subtle, but interesting optical effects.   I am playing with better keyboarding habits -- I hope the formatting holds for this post.

Marian Gooding Neon-Cat Ceramics www.neon-cat.com http://www.flickr.com/photos/neon-cat/

--- On Wed, 1/25/12, Bill Merrill <BMer...@pencol.edu> wrote: