Phil Hamling

376 County Route 1

Warwick, NY, USA 10990

e-mail: (change the -at- to @)

Molybdenum Crystalline Glaze

A chronicle of my recent progress and a way for me to keep it straight in my head!

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 Info I've been asked to keep in confidence.

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Crystalline Glaze Info

Electric Reduction Symposium

Friends' Work

I set this page up to facilitate collaboration between people who have the ability to contribute and benefit from shared ideas, trials and results. those listed below have access to this page. The main rule is that you share all. This is the e-mail I sent to everyone.

Joerg Baumoeller Gordon Czop John Dunlavy John Tilton 


10-2-12     Joerg's thoughts on Rita Bassett's article     Moybdan_essentials.doc

7-26-12  No titania!!!!

Mixture #1 Mixture #2
Add to 100
Base Solids
Add to 100
Mixture #1 Solids
F3134 31.72    
Lead Bisilicate 4.53    
Whiting 18.13    
Silica 12.39    
EPK 18.13    
Add 50% water      
Calcium Molybdate   4  
Black Copper Oxide   1.5  
Cobalt Carbonate   .75  
Manganese Dioxide   .75  
Gerstley Borate     5
Glaze Load (g/ 1.0
1080C Hold (hr.) 1.5    


      Mass (g)
Gordon's Base      100
F3134   5
Calcium Molybdate   4
Rutile   2
Black Copper Oxide   1.5
Cobalt Carbonate   0.75
Manganese Dioxide   0.75
Glaze Load (g/   0.5
1080C Hold (hr.)   4

7-22-12   I just took these out this afternoon after firing with a 4 hour hold. The pieces with 6% and 8% rutile are bizarre and don't fit the table. I'll wait until the 1 hour and 0 hour hold firings are finished before putting up photos. I would swear I screwed up the formulas except I made the first one shown below of the same materials.

  No rutile, less: copper, cobalt, manganese & iron plus 1/2% tin oxide.    


 I sent out an e-mail to the group showing the results of my test from 7-18 on 7-19 and later the photos shown below under 7-20-12. I received comments as follows: Gordon Czop, Joerg Baumoeller, John Tilton.


The basic explanation is I batched the following, which I have been referring to as Gordon's Base. For a detailed explanation of compositions see "gordon's glaze analysis 7-21-12.xls"

Mass (g)
F3134 210
Lead Bisilicate 30
Calcium Carbonate 120
Silica 182
EPK 120

Then mixed the ingredients in a blender with 340 ml water and 1 drop concentrated Epsom salt solution. (The ES functions a de-flocculent in these small concentrations as opposed to as a flocculent when added in the 1 to 2% range.)

To a 151g sample of this base glaze, which contains 100g of solids, I added the following colorants.
Sample Name G1 4% Rutile G2 0% Rutile
Rate Mass (g) Rate Mass (g)
Black Copper Oxide 1.5% 1.5 1.5% 1.5
Cobalt Carbonate 0.75% 0.75 0.75% 0.75
Manganese Dioxide 0.75% 0.75 0.75% 0.75
Calcium Molybdate 4.0% 4 4.0% 4
Rutile 4.0% 4 0.0% 0
then blended equal amounts of G1 and G2 to get what I called G1/2 or 2% rutile

I applied these glazes to 4" square tiles at an ~1/2g / loading. The "math" goes something like John cited "..... when you calculate the amount of dry glaze that should go on a tile, the calculation is 16 x .5 x 1.51 = 12.1 grams of glaze per tile." I targeted 12g per tile. For each tile I placed it on a scale and zeroed it. Then I put the tile on a banding wheel and applied an even coating until I got to 12 grams. I kept track of my progress by periodically putting it back on the scale. I was able to hold the loading in the range of 12 to 12.5 grams in most case.

I prepared 3 tiles of each formulation at a 1/2 g/ loading as well as one of each formulation at a 3/4 g/ loading.

I fired them using the same schedule except I varied the 1080C hold time from 4 to 1 to 0 hours. The basic schedule and typical firing curves are shown below.

Firing Schedule: Segment, Ramp (C/hr), Temp (C), Hold (min.) 4 Hour Hold 1 Hour Hold 0 Hour Hold
Segment Rate (C / min.) Temp. (C) Hold (min.)
1 200 1171 10
2 150 1305 0
3 340 1050 0
4 240 1080 240


7-20-12 Effects of titania content and hold time.

7-18-12      Gordon's glaze - 4 hour hold.


7-15-12   Gordon's glaze with 1.5% black copper oxide, .75% cobalt carbonate and .75% manganese dioxide and varying rutile content applied at a rate of 1/2 g/

Firing Schedule:
Ramp (C/hr), Temp (C), Hold (min.)
200, 1171,10
150,1305, 0
340, 1050, 0
240, 1080, 240  
Revisiting one of Herbert Sanders' ^4 recipes and trying to get the moly content right.      
The crystals and iridescence are almost impossible to see unless in reflected white light This is the first time I've seen any crystals come from one of Sanders' recipes. At this glaze loading 3% MoOx seems to be the trick.   It looks like 4 1/2% red copper oxide, 1 1/2% cobalt carbonate and 1 1/2% manganese dioxide is more than this wimpy ^4 glaze can digest.
This test has samples with a line blend of from 0 to 4% molybdenum oxide


7-3-12  Ulrike Franck left the group with this e-mail.



  From Joerg......Phil... did you fire these with the "Gordon's CaMoO3 glaze" 5/10/2012?
F3134 27.70
Lead Monosilicate 3.96
Whiting 15.83
Silica 24.01
Ball Clay (OM4) 15.83
Rutile 3.17
Powellite 5.07
BCuOx 1.58
CoCO3 2.86
Total 100.00
Reply from Phil.......Yes. After recalculating to 100% with all the additions the recipe used was as appears on the left:



I thought I was going to fire this without the 4 hour hold, but screwed up. It went through with the hold.  I added 3% cobalt carbonate to Gordon's glaze  The cobalt resulted in smaller crystals much like it does in zinc silicate crystalline glazes.  Psycho 
Line blend containing 4%, 6% and 8% powellite.  4% powellite.   6% powellite.   8% powellite.  
It would be nice to get these to where they are a little less blah and speak to the viewer from across the room.       


From Gordon......:The process for roasting MoS2 to MoO3 is done the same way (without CaCO3) as synthesizing powellite.The sparkles you see on the plate of powellite are probably some MoO3 (looks like needles) that sublimed and recondensed on the surface, see photo of roasted MoS2>MoO3. John..On Phil's blog, a while ago, there was a picture of MoO3 and WO3 that Gordon had roasted. Would this have been done in the same process as roasting the Powellite, but without the Whiting? Can you just put MoS2 and WS2 into the plate, fire it up in the same way, and end up with MoO3 and WO3?


From Gordon......:Same firing schedule and glaze constituents except as noted. The dome piece was fired in a saggar and the 100 gram cup in the open,  4 hr hold, Third firing for the dome:1st: 3% Powellite10% Manganese Carb and 2% red CuOx. One crystal in the drippings on underside of piece. 2nd: 5% powellite. Completely covered with a sheet. 3rd: 4% powellite, 2% Manganese Carb and 1% red FeOx. Cup: glaze same as 3rd fired dome, 0.6 g/si Gordon  

  From Jon Dunlavy....try with 6% titanium diox, 4% Moly triox. I color most of my tests with about 4% cobalt ox. Frit 3278 is also worth response to John Tilton writing ..Now I need to decide how to spike them up so that there is a chance for some Mo Crystals. My thought is to add 4% Molybdenum and 4% Rutile and a bit of color, say less than .5% CoCO3. Just enough for some color, a nice light blue. Does this seem intelligent for a start?

From Ulrike......Phil, bismuth is strong flux, even more than lead.....Theoretically you could use it to substitute the lead completely. Bismuth is the flux part of lustre overglazes, Feri surely will use it. And as lustres are reduced, this surely will work. But for me it is interesting in this context, because it also reacts with molybdenum and crystallizes in the tetragonal system. But the two ......are details from the tall bottle. Two glazes overlapping, the black one with lead and no Bi2O3, flowing over a white one, no coloring oxides added, with bismuth. Bismuth crystals seem to be smaller - but I guess they just need another firing cycle ...Maybe they crystallize best at another temperature/holding point.  

5-21-12     e-mail with thoughts from John Dunlavy.

US Patent 2663658, Ornamental Crystalline Glaze From John Tilton

5-19-12   I came up with some recipes to test based on Ulrike's formulae using GlazeMaster software. This was kind of frustrating at first because it did not generate the exact same numbers as did the Online Glaze calculator. I recalculated Gordon's 3134 recipe with it and started from there.

5-18-12      e-mail reply to John Tilton's "Swimming out to the Ship" with thoughts from John Dunlavy.

e-mail reply to John Tilton's Twisted Crystals with thoughts and suggested formulae from Ulrike.

  In it she wrote " One more thing to the Koerner: I have an article written by prof. Ralf Busz, where he is mentioned. mentioned in a context of mo glazes with Bismuth - not B2O3. Therefore I thought the B2O3 would be just a printing error and calculated the recipe without it. Bismuth is very interesting in these glazes, I found."

 Metallurgy For Dummies - Bismuth Alloys

Twisted Crystals e-mail from John Tilton.

5-16-12   In this test I took the glaze from 5-11-12 (copper and rutile) and added 1/2% each of cobalt carbonate and manganese dioxide, then used the same firing schedule.

I added extra powellite by eye ("EPBE") (~10%???) to the last bit of glaze and applied a band to the top of a previously glazed vertical tile where an 1/2 g/ loading produced no crystals. It was nice to see this in the bottom of the kiln. This tile with glaze loaded at 1/2 g/ produced a crystal / background ratio very similar to to the initial trial without cobalt.  
All the crystals from the last 2 firings seem like the crystals formed then started to break apart. I wonder if they formed during the initial descent from top temperature then were changed during the "hold". It looks like the lattice has shifted (grown in one direction and shrank in the other) splitting them apart. The 4" tile on top was a refire of a piece previously glazed with "P2A" (see note below) with EPBE added at a rate of ~ .2 g/

The 4" tile refire, previously glazed with "P2A"  with EPBE added was completely covered with crystals on the half originally loaded with 1/2 g/ P2A and less densely covered with ~1/8" square crystals on the half originally loaded with 1 g/

"P2A" was made of the formula I show on 4-22-12 where I commented "more closely match Pukall's unity formula numbers using materials I have" with 2% molybdenum oxide added. It produced no crystals during the first firing whether applied at loadings of 1/2 or 1g/   Its formula is shown below.

I found these 2 abstracts online via Google this morning. It looks like they contain some very useful information.

5-15-12   Joerg....30 years for Bayer, but i am not a chemist... I think your dark black powder is probably molybdenum disulfide

Photos from Joerg



Consistence Glaze : liquid-creamy
brushed : middle thick, 1-2 mm
all on  porcelain plates Vicente Diez, Spain,
 bisqued at 1100°C , 10 cm x  15 cm x 0,5 cm

“S7” = “MA #4 Glaze” cone 9, page 44 Sanders book. (Hommel Frit replaced with Frit 3110).

“S4” = “MA #1 Glaze” cone 9 , page 43 Sanders book

FS  S4 + S 7 
in 7,5 h to 1260° C, 15 min hold
in 1 h to 1110° C, 5 h hold
in 1 h to  1060° C, 1 h hold




Out of 11 formula proposals from Ulrike3”, “4”, “6” showed  (tiny...) crystals in my kiln... 




K/1  K/2 FS:
 Lead Mono Silicate   45,7 44,2 in 6 h to 1250° C, 20 Min hold
Frit M1233  21,9  21,2 in 35 Min down to  1150°C , 10 Min hold
potash feldspar       5,6  5,4 in 5 h down to 750°C
Caolin     3,9  3,8 End
Whiting 9,1  8,8
 Flint   10,8 10,4
  Consistence Glaze : liquid-creamy
3.) K/1 with 4% black Molybdene oxide  (moly disulfide??) brushed : middle thick, 1-2 mm
4.) K/1 with 4% black Molybdene oxide  (moly disulfide??) + 4%  Bi2O3 all on  porcelain plates Vicente Diez,  Spain, bisqued at 1100°C , 10 cm x  15 cm x 0,5 cm
6.) K/2 with 4% black Molybdene oxide  (moly disulfide??) + 4%  Bi2O3




Phil : Thanks for the notification.

Just chiming in to agree powellite is not absolutely necessary and cone five and hotter is effective.

(Synthesizing powellite from MoS2 evolves SO2 and/or SO3, very nasty) Gordon


John.....could make sheelite (calcium tungstate) by the same roasting process as with powellite?

5-12-12  Short Wave UV 
Left to right: 1g / and  1/2 g / glaze loading.       
  Short Wave UV    Short Wave UV 


5-11-12    It appears that "holds" are not needed in the firing schedule to promote the growth of Powellite crystals.

The piece, by Gordon, on the right, fired in a saggar, suggest a slow cooling, helps grow larger crystals, These used the formula and firing schedule, listed below.  The pair on the right is colored with 2% MnO2 and were fired using the previous schedule with only a 1 hour hold. The saggar fired piece has the largest crystals yet, It weighed a little more than it's counterpart on the left, suggesting further that a slower cooling from the top temperature promoted the growth of larger crystals. 
The 6" square tile was fired on the floor on top of a piece of ceramic fiber insulation.  These crystals seem to crack apart as they form.     
I made a series of vertical tiles with 3/4, 1 and 1 1/2 g / sq. in. loadings. The first and last formed crystals on one side only. The 1 g / loading formed virtually none.    I believe the crystals grew larger partly because it was insulated and cooled more slowly.  Powellite, Black Copper Oxide and Rutile. 
  Firing Schedule:
Ramp (C/hr), Temp (C), Hold (min.)
200, 1171,10
150,1305, 0
340, 1050, 0
240, 1080, 240  
This formula had 4% rutile, 4% powellite and 2% black copper oxide added.   Results  


4-29-12  Fuming

Fuming   5-4-12 Bill Campbell wrote "Did you dissolve your SNCl in alcohol and water? If that is sprayed into the kiln, and not directly on the pieces, you should be able to watch the luster develop.If you have an old piece that has silver in the glaze, try that. The tin really likes silver, and even better the combination of silver, and cobalt. A little reduction after the fuming helps.

4-28-12 Making Powellite

 I mixed molybdenum trioxide (MoO3) and whiting (CaCO3), screened them 2X, then fired to 800C and held for 1 hour to produce powellite (CaMoO4). I ground it in a mortar and pestle then passed it through a sieve.

Gordon  roasted 100g CaCo3 +160g molybdenum disulfide at 760C. It still showed some signs of sulfur, but can be stirred and reheated to complete the reaction. See 2-4-12.

MoO3 - CaCO3 (right) converted to CaMoO4 (center) Powellite fluoresces under short wave UV light in crystals on pottery as well as in powder form. A Herbert Sanders recipe with 4% and 8% MoO3 added.

The iridescent areas and "egg yoke" colored eruptions of the Herbert Sanders samples both fluoresce yellow indicating they are powellite. I think the next step would be to reduce the moly content.

Gordon and I spent the afternoon discussing molybden glazes and comapring reslts. He later wrote: Phil, Bassett says two or three times  that the crystals flow with the glaze and collect in areas.Gordon

4-27-12      Pawellite Powellite

4-25-12     Phil, The translation was helpful. Thanks, Gordon 

(I sent Gordon this section from "My experiences with crystal glazes / part two", Dr. W. Pukall, Sprechsaal  - Nr. 37 / 1908 Translation  Ulrike Franck)

new over old plate fluid glaze . stiff glaze from article on today's pot   didn't work no crystals
on neck put over glaze wit powelite. 40 -3134, 20 ea (wollastonite, epk + silca) + 3 -5% powellite
100g caco3    160g moly disulfide

make powellite, put it in the glaze, has to be  a glossy shiny glaze
crystalline glaze firing


4-25-12      Development and control of molybdenum crystals in a stoneware glaze Bassett, Dorothy Rita 1949-, 1978 MFA Thesis, Universtity of Florida This is a 7MB file

Note pg 47 for discussion regarding fuming to enhance and intensify iridescent qualities of molybdenum crystals  


4-22-12 Analysis of molybdenum glaze recipes

4-12-12     Gordon's thoughts on molybdenum glazes. Phil, I hope these notes are helpful, my experience,  attempted to be presented in some coherent, useful fashion.Your thoughts on improving it in any way  are welcome. Thanks, Gordon

4-11-12 E-mail with Ulrike  Intro and reference to technical article by Koerner.


Complete Book: Transactions of the American Ceramic Society Volume 10




3-18-12 "Molybdenum" crystals are illusive to say the least. Plus anyone with a clue has zipped lips.

Gordon grew these crystals from a pellet of powellite placed in the closed space between 2 plates.    
UV suggests they are calcium molybdate crystals.   .A Herbert Sanders recipe with 4% MoO3 added.


2-19-12    Bulldog Pottery.....Iron red molybdenum crystalline


Gordon, The test results of the 10% MoO3 and WoO3 in crystalline glaze and liner glaze was bizarre. The liner, which really never runs, reacted and ran off the tiles. MoO3 moved the most and made streamers down the tile. The crystalline glaze, which always runs, didn't move a bit. I'll try and get some photos tomorrow. It's nap time now.



Gordon, This was after firing to 850C, 1 hour hold, cool 150C/hr to 650C. I think I'm going to stop this one here. These crystals just look happy growing on the surface. The other tiles with 10% MoO3 and WoO3 in crystalline  and non-crystalline glazes will be out tomorrow. I think I'll look at loading  a crystalline glaze (without zinc) with calcium carbonate and MoSi2. Phil


Zinc Molybdate, SiO2 - ZnO2 Phase Equilibria, Calcium Molybdate, Calcium Tungstate, information.


Phil: I ran an experiment with a high calcium glaze  and separately the components for CaOMoO3. Bottom line: nothing. Glaze 3134 37%, epk and silica both 20%, whiting and wollastonite both 10%. Firing to 1160 and a three hour slant soak to 1060. Next will try free MoO3 and the glaze (as originally planned), perhaps some moly in the glaze on another piece as well. Regarding those latest photos I'm glad that was glaze and not some alien biological entity. Gordon


2-9-12 I repeated the previous tests using crystalline glazed tiles at up to 950C? and saw some deposition or crystallization on the surfaces. I did not want to go so high in temperature as to start taking the zinc silicate crystals apart. I failed to keep geed notes on the exact temperatures. I could have stated exactly  what they were during the time I was performing the tests but abandoned the effort when it appeared this was not the correct path and failed to record them.



Gordon, I did another test in which I put 2 pre-fired crystalline glaze samples in the saggar with 1/2g MoO3 powder on each catcher. After 1 hr. at 800C it seemed that only ~ 1/2 of the stuff went poof and I got this.


2-7-12 Samples were fired in the saggar shown below. The thermocouple showed that although there was an ~50C difference between the inside temperature and the kiln temperature on heating the difference was minimal during holds in the 1000 to 1200C region. At 1000C with a 1 hour hold there some volatilization of moly. I did not see much reaction with the glaze if any. After firing higher (to 1100) I saw the moly materials completely volatilize and a significant matting of the glazed surfaces. There did not appear to be any deposition on the bare porcelain surfaces or saggar walls. The glaze took on a bubbled appearance. I surmise that moly got into the glaze at the lower temperatures, but at the higher temperature (1200) began to form bubbles under the surface, but was "frozen" in place upon cooling preventing it from boiling out.


After up to 1000C - hold 1 hr - cool to RT, then up to 1100C - hold 1 hr - cool to RT and finally up to 1200C - hold 1 hr - cool it looks like something got in the glaze that wants to get out in the worse way.



I don't remember if I told you or not but I ordered 2 1/2 kg of Molybdenum trioxide (molybdic anhydride) from this company. I got the attached MSDS today. I am surprised by the info in the Physical Properties section, specifically the melting point and boiling point data.


Gordon., I fired my test setup for 1 hr. at 1000C (1832F). All the moly oxide and metal vaporized – went poof. I do not see any on the inside of the crucible, on refractories or on porcelain. The bright shiny test pieces took on a matt appearance and seems to have a shiny haze on it as if there are many, many little molybdenum trioxide crystals on the surface. I sent it back up for 1 hr. at 1100C and should see the results tonight.    Phil

2-5-12    RE: Trying to understand molybdenum crystals, e-mail dialogue with Halmos Ferenc.

Gordon wrote: Consider MoO3, it  has a documented vapor pressure curve. See attached spreadsheet from CRC handbook data.


I have this setup in the kiln today. It will get to 1000C at ~ noon. Do you think I should fire it higher? The MoO3 vapor pressure curve shows that it has all sublimed at ~1150C, but I wonder if I really need to go that hot. Kind of like water evaporates pretty significantly below 100C.

These photos show 2 different ^10 glazed tiles, with a fair whiting content. I've added 1g MoO3 powder and a few square inches of scrap molybdenum (metal) screen to the mix. I have covered the setup with an inverted alumina crucible to serve as a saggar and included some alumina fiber paper as a gasket. Also there is a thermocouple inside the saggar so I can get a handle on the inside temperature as well.

What do you think?




2-4-12   Re-firing test pieces with calcia containing glazes in an attempt to fume molybdenum oxide vapors. An alumina crucible was used as a saggar. Samples of molybdenum wire and molybdenum oxide were placed on the catchers on the bottom.

2-4-12 Discussions with Jon Dunlavy

"Molybdic oxide crystallizes on the surface of the glaze and they are not grown like other crystals with a slow cooling." "Molybdenum trioxide is the usual source"

2-4-12 Some interesting information on water crystals

Elementary Meteorology Online
Chapter 7 - Clouds, Precipitation, and Weather Radar


Here is an example of what you were mentioning(taken before the october snowstorm).

Here's a combo where the crystals fluoresce yellow.



Jon does have some interesting stuff going on here.

I plan to re-read Sanders' book today.

I was real interested to review the MoO3 vapor pressure curve you supplied. I wonder how it compares to that of zinc oxide. Do you have the CRC Handbook and the ability to look this up? I've Googled "zinc oxide vapor pressure" and see there is a lot of information out there, but one would need to buy a bunch of books or belong to a university to get to it.

I'd love to work with you to develop a molybdenum glaze. How do we get started?

I batched up some of the material you gave me with a crystalline glaze and whiting / Custer feldspar based one, at a 5% addition level, to try getting started by using the "poke and hope" method. I'm not sure where this will go.



Here's more food for thought



Here are MoO3 crystals formed by roasting MoS2 at 760c. The reaction is exothermic so initially the MoO3 is hotter, it sublimes & then cools and condenses to form crystals.

They do not fluoresce;they are 3 dimensional needles. Am close to being convinced the crystals folks grow in glazes are CaOMoO3 which would fluoresce yellow.

If so perhaps the MoO3 condenses onto the glaze surface, interacts with CaO  in some form and starts the crystal.

My next step will be to make a  cone 3-4 glaze with calcium( but no

lead) in it, fire it alone first, put it in a saggar with MoO3 separate but enclosed, heat to 1150 c or so and then do a very slow cool to about 1000c.

Thanks for the fascinating information on water/weather, maybe I'm too easily convinced but it seems clear that something like that is happening in the moly crystal glazes.




Thanks for the info.

Today at work I heated some pieces of moly wire and sheet slivers with a torch. They seemed to glow hotter than the surrounding refractory and put out a thick yellow -white smoke. The reaction and glow stopped when I removed the heat but the smoke kept coming for a little while. I think the risk of explosion is minimal.

I brought home some alumina crucibles which I will use as saggers and heat up some stuff. I'll photo the setup and any results worth the effort.


Does anyone know Jon Dunlavy? I found this photo on Flickr after searching for "Molybdenum Crystals" Here's another of his pieces I found on someone else's Flickr account.
Then I found his website but didn't see any molybdenum crystalline glazes.

2-20-2011   Moly crystals by Bulldog Pottery.


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Recently I had the good fortune of importing many very interesting crystalline glazed pieces from Beijing, China.

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