View Full Version : Phosphate Opal

Rich Federici
12-03-2006, 09:20 AM
After reading an older thread on Phosphate opals, I tried the following:

Quantities in pounds, except as shown

Sand 20
Soda Ash 4.6
Hydrted Lime 2
Zinc Oxide 1.2
Borax 3
Alumina Hydrate .6
Lithium Carb .6
Potassium Nitrate 73grams
Sodium Tripolyphosphate 2.8

After melting, I got a lot of crap on top but the glass underneath seems to be ok. I could have melted hotter, but wanted to take it easy because of glass in the other 2 pots.
My question is would a change to the time and temps of the melt help to reduce the junk on top, or is this just the way it is with this particular formula. The original purpose was to have a white that would be fairly dense, and have a viscosity close to the other colors that I make would have.

Pete VanderLaan
12-03-2006, 10:02 AM
Phosphates are not really dense glasses. They can and must be melted hot 2325F or higher. The junk will diminish is you melt hotter but it's almost always a problem in cooler melts.. Screen the phosphate materials. You can go up to 5% phosphate before you will hit a compatibility wall. Borax helps.

Rich Federici
12-03-2006, 03:18 PM
Thank You Pete,

I will see what it looks like tomorrow, Maybe sitting all day helped to fix it some.

The neat thing that I see with the small pieces that pop off the punty is that when you hold them up to the light, you can see that reddish orange glow near the the thinner edge. It looks like they are lit up from the inside.

Hugh Jenkins
12-03-2006, 05:46 PM
Also be absolutely sure you are oxidizing. If you are gas melting, the flame can often be partially reducing at the core even if there is total combustion in the fire box. A really clean burn is needed for phosphate.
Certain colorants that are bi or tri-valent seem to help in maintaining a little longer stability of the phosphate. Manganese at .1% is great for this and won't effect the color. Copper and chrome additions will create color but also extend the life of phosphate opals in the pot. Otherwise it is necessary to get them out within a day or so.

Kenny Pieper
12-04-2006, 08:46 AM
Also dont let it sit too cold. The chuncks will come back. I have found that there is a fine line when melting to get it into solution and when the opal starts to weeken or burn out. Takes lots of heat like Pete says

Rich Federici
12-04-2006, 09:53 AM
I removed the scum from the top and have made a few test pieces. A simple thread test stayed pretty straight. Incidentally, I ordered my Spruce Pine 87 a little too late,having run out, I made my own clear glass.

One was a cylinder to cut and check as a ring test piece, it is in the annealer and I will see tomorrow. One thing that I noticed on the trim scrap was the white glass still seems to have a rough texture.

It looks ok cased in clear but by itself, not too smooth. It almost looks like tiny bubbles that when you feel them exhibit a rough texture.

Am I too picky? or is that what this is supposed to look like.

You guys were right in that it seems less dense than the cryolite white.

I like this two furnace thing but may have to put the brakes on for a while and shut one of them off. The latest gas bill topped $1700.

Thanks for all of your help.

Pete VanderLaan
12-04-2006, 03:48 PM
Originally posted by Rich Federici

I like this two furnace thing but may have to put the brakes on for a while and shut one of them off. The latest gas bill topped $1700.

Thanks for all of your help.

I'm proud of you bucko! I love it when I see people using the class.

Rich Federici
12-04-2006, 04:17 PM
This afternoon we pulled some white stars cased in dark blue.

The white was the phosphate opal, and the blue was cobalt oxide tinted SP87.

The phosphate white is definitely not as dense, but... the star seemed to hold its shape better.

The murrini also seemed to cut better than the cane we did a while back with the cryolite white, which some split as we were trying to cut them.

The second part of the test is to see how sharp or how distorted the murrini become on the surface of a small blown piece, like the size of an ornament.

Now looking at the 2 stars side by side
at this point I might have to vote for the Phosphate opal as far as keeping its shape but we shall see tomorrow.

Pete, I really got a lot out of the class and now I finally have all my ducks in a row enough to put some of what you taught us in practice.

Rich Federici
12-04-2006, 04:21 PM
Pete or Kenny,

First, Thanks again, and second, what kind of melt cycle might you run for the above formula?

Since hot is key, how hot do you think, and how long?

Pete VanderLaan
12-04-2006, 07:02 PM
no less than 2300F. Screen it well. Small charges. Proof after each charge and hold the charges until the proofs show no unmelted stuff. The path to phosphate opals leads to madness.

You are not going to like this stuff for murrinis. It is not dense enough. Incredibly beautiful as a stand alone. The viscosity will be very similar to your basic clear.

John Croucher
12-05-2006, 01:55 AM
P2O5 over 5% is pretty much asking for trouble. Your recipe comes out at 5.3%. Back off 15% on the STP, substitute for the soda content you have lost and try again. Redox is irrelevant in my opinion (sorry Hugh) -its all about the devit temp of the apatite crystal/globule you are creating at temp within the matrix.
Colorants may change the devit temp, so Hugh's advice has some relevance.

Pb brings down the devit temp if you want to go down that route. According to Weyl a little F2 stabilises the apatite emulsion (think teeth)which is why the old bone ash recipes seemed to work better. A lot of the 19th century recipes used guano (sea bird shit) and incorporated a little Greenland cryolite or CaF2 in a hydrid P/F recipe to accommodate the difficulties of a pure P opal, even though guano probably had suffient F at temp.

Animal bone ash contains F, in maybe the right stabilising proportion.

Pure phosphate opals are a route to madness but can be conquered, albeit with quite a bit of experimentation..

Don't expect a dense white, but phosphate opals are good for stiffness and are user friendly re pollution. There is more color play with a reddish transmission and there are a whole heap of color combos. They certainly need to be melted and worked out hot to stop devit. Theoretically they shouldn't fade with time at temp as against F opals, which do lose opacity owing to F volativity, but you can never know for sure with colored glass making.

The bottom line -opal phosphates are one of the last exciting frontiers of studio glassmaking.

Hugh Jenkins
12-05-2006, 03:06 AM
John, Great to read your contribution on this. Devit and phospate concentration are the main issues. But from experience in melting, the flame settings did make a lot of difference. One reason that most clear batches cannot be made into phospate opals is the presence of antimony. Since antimony acts as an oxygen absorber in the cooling phase, it can reduce certain less stable oxides. Identical oxide formulas without antimony were way more successful. Keeping the phosphate down in the 4 to 4.5% range gives beautiful results.

Pete VanderLaan
12-05-2006, 06:08 AM
Bob Held did some good work on this back around 1973 and demonstrated that up to ( not exceeding) 5% phosphate would still be compatible. I did melt it and it did work and it was troublesome. We usd this old stuff mark Peiser had called Bone Ash 21 which is long gone. It was a genuine bone Ash, not a synthetic. . I do think that if you use Sodium triphosphate, it would help to back off of sodium somwhere else.

The devit is clearly an issue and I agree with john that lead would help here but it is lead...

And John, obviously we got around your posting problem on the board. Good to see you here being my fussy conscience.

Dave Bross
12-05-2006, 12:01 PM
Many thanks to John for participating too.

Here's the original thread on phosphates from here:



Here were my last discoveries on phosphates before all the turmoil that shut me down.....

Sodium tripolyphosphate ended up being the best phosphate source for the least amount of chunkiness. It's a common ingredient in cleaning compounds and easily available at the chemical supply outfits.

The amount of calcium has a large effect on the density and strike of the melt, more calcium seems to equal less strike/density.

Viscosity does seem to be a key issue. I got a fairly decent and quite dense 6% phosphate melt in the last melt recipe listed in the old thread with more lithium and zinc. The low viscosity melts strike quite a bit right out of the pot. They also digest your pot.

I used 1.5% alumina in the melts but in retrospect I wonder if that was adding to the chunky/devit problems. I would think all the zinc would substitute nicely for the durability enhancements alumina brings but Volf warns that this is not so. Unfortunately, Volf didn't explain, just listing a reference to another publication.

I decided to stay away from fluorine due to the electric melter but after reading John's take on Volf I think I'll at least try to match what might have been in the original bone ash.

I'm stuck at low melt temps. with the wire melter but I would definitely like any suggestions on lowering the devit temp. other than with lead. I suspect that the high zinc percentage helps that? Time to go trolling in Volf again.

I think the low toxicity of these glasses will be important in the future.

along the lines of lower toxic glass...

I'm curious about the old glasses using talc (magnesium silicate) for an opacifier too. Probably not much more than an alabaster sort of opaqueness due to the 5-6% limits on the talc.
I tried a tin opacified glass and it was terrible, so short it wasn't workable.

Hugh Jenkins
12-05-2006, 12:50 PM
When I was teaching and any time I have been at Penland, we used Spruce Pine. The color base for SPB is a good match but holds out the decolorizers, fining agents and oxidizers. We were able to make better phosphate opals with that than we did mixing our own batch. I think Labino covered a lot of things with the formula that SPB is based on. It has small amounts of many things that just seem to help. The zinc, boron, flourine and barium are not enough to really change the glass much but they are just right for selenium-cad colors, phosphate opals, fairly saturated transparents, holding silver and gold in reasonable amounts, etc. I have actually come to like other clear formulas better for working properties, but I think that for most color making, this is a great basic glass.

I'll dig around and see if I can find the notes on that mix. In order to compensate for the sodium in the STP we had to make almost complete glass with the STP and add it in proportions to the color base.

Henry Halem
12-05-2006, 03:40 PM
As an aside to melting phosphate opals here is a short story on Peiser's adventure with Phosphate opals in the early 70's. Mark figured out early on how to melt really nice phosphate opals. His adventures with phosphate opals began during the spring and summer down in NC. He developed really fantastic colors but for some od reason when winter came (or the other way around) the same phosphate formula that worked so well during the summer would not work at all during the winter. It drove him buggy. Eventually he came to sort of believe that the difference was in the moisture content of his chemicals.

Dave Bross
12-05-2006, 03:53 PM
I too think there is something to that. I've melted with and without wetting down the batch and the wet went way better.

A side note on that... Hugh had told me about using a tiny amount of manganese to oxidise it. When melted dry the opal was white, when melted wet it would be a very pretty rose color, which says to me a radical change (increase) in the oxygen content of the glass, bringing the manganese into the valence state showing max color.

What Hugh mentions about compensating batch and the amount of sodium in tsp is another possible reason to use the sodium tripoly phosphate. The stpp is .42 sodium and .58 phosphorous, the tsp is .24 sodium, .19 phosphorus, .57 H2O.

I think Nick's glass is a great formula too, with the exception of too much lithium to work as a lampworking glass. I shouldn't really complain about that either, it was never intended for lampwork when it was designed.

Rich Federici
12-05-2006, 04:39 PM
So the white pieces that were cased looked real nice. I made a pineapple mold ball from just the white and it feels like sandpaper on the surface.

Perhaps I did not melt hot enough, and I probably put too much in at one time. These are things that now I know I can fix I also will have to balance which colors and how I charge with regard to doing the hotter ones first.

Thank you John Croucher and to everyone that chimed in on this project. I will go back and study some more, hopefully to make this better next time.

Pete VanderLaan
12-06-2006, 05:30 AM
when I try to make it better, lots of times I just make it different. We call it another triumph of hope over experience Pay attention to lowering the phosphate content as John Suggests. 4% is nice.

Dave Bross
12-07-2006, 04:16 PM
One more thought... you can repeat cooling and heating (restriking) the lower percentage phosphates for more density. The glass gets quite stiff and it is time consuming.

Pete VanderLaan
04-11-2009, 06:18 AM
I am getting ready to try making some phosphate opals and in going back to this thread I found it to be so useful as to be worth bringing back to the front. I will eventually move it to Antiques and Classics.

I have acquired sodium tripolyphosphate from Univar in Providence which was kind of pricey at $1.80 lb. If dave is reading this I was wondering how carefully you screened, or crushed the stuff? Chunkyness has always been a big issue with these glasses. I am by the way finding that a small coffee grinder is a fabulous way to crush small quantities of chemicals.

It makes the coffee taste funny though.

Rich, in looking at the formula you originally used for this thead, . What L.E.C. were you aiming at with that glass? Was it an SP compatible?

Rich Federici
04-11-2009, 12:49 PM
Dear Pete.
when I melted this, I found the sodium tripolyphosphate at a place called the chemistry store online in 50 lb. bags, at about $2.00 a pound. They sell home soapmaking stuff.

I made star murrinis with cobalt blue from regular SP87, no splitting occurred.

I only tried it one time, and the crap floating on the top was a real issue for me. I hope to re visit this again because the fire opal possibilities really excite me.

Incidentally, the bag of sodium tripolyphosphte says "for use in water purification systems" , a plus!

Pete VanderLaan
04-11-2009, 12:56 PM
Did you really crush the stuff? Or did you just add it. Screened? what?

Also If I try it your way i will mix the lithium and the polyphosphate together if you remember that particular trick?

Dave Bross
04-11-2009, 02:41 PM
Here's a link to an old post of mine explaining "the fix" for chunky phosphates:


Here's the text from there to save everyone a trip:


I've been trying to get a phosphate opal without the "texture" for a while now. The answer turned out to be mainly in the modifiers.

You don't use more than 3% calcium, 5% strontium, or 9.5% barium while using at least .4 - 1%, but not more than 2% zinc to help retard the formation of the dreaded surface crystals, and 2-5.5% phosphorus.

I (finally!!!) found all this in an old Corning patent, US patent # 4536480. I've done some melts with the calcium, strontium, zinc and phosphorus (no barium) in those ranges and it works beautifully in terms of no surface ugliness and the usual great beauty of phosphate opal.

The patent also suggests 1.5% fluorine to get a little apetite cryatalization going, but as a wire melter kind of guy I left that alone, and I'm quite happy with the opal I got without it. a bit of reading between the lines suggests you will do fine without the fluorine, making...

"glasses wherein the opal phase is essentially non-crystaline; i.e., the opacity results from the presence of immiscible glass phases"

After all those melts of "sandpaper" phosphate opal I think I'll pass on any kind of crystals in my phosphate melt.

Obviously, you would have to batch this one from scratch because no stock batch goes that low in all those modifiers.

I like sodium tripolyphosphate for a phosphorous source. It's pretty common at chemical suppliers, it's used in a lot of cleaning formulations.

Just screen the STPP and it will be fine. It really almost doesn't even need that but I do it just in case there's something unexpected in there.

Don't worry about contact mixing lithium with it, it will go down just fine as-is. After everyones struggles with getting phosphate to melt you're going to be surprised at how easily this melts.

Y'all are just going to LOVE what you get if you go to the trouble of batching these. Colors are amazing done up in phosphate. So much more stable and less toxic than fluorines too.

Pete VanderLaan
04-11-2009, 03:21 PM
well, I want the opal and I have a five pot moly. I have to build a little gas unit for the fluorine opals anyway but it would be really nice to not have two furnaces on at once these days.

I have to confess that what you have written in terms of formulation is utterly incomprehnsible to me the way they are written. Do you mind putting down a basic formula (without strontium, which is one of your favorites i know but I don't want to get into another bulk item in the batch room ). I already get the notion that this is an emulsion glass and the phosphate glass is embedded in the silicate glass. What i see you saying is to not go above 3% Ca and 2% Zn. That's 5%. Are you advocating that to be the max of the network modifier?

Spit it out.

Dave Bross
04-11-2009, 07:52 PM
Actual batch specs in a minute...

Now Pete, you know you raised me up better than to go around short on my modifiers.

I'm targeting 8% or greater total modifiers. That's correct about staying within the limits mentioned in the original post. Strontium or Barium are also modifiers and make up the difference to get up to the 8% total. I'm guessing you stock Barium. That could replace the Strontium. You're going to have to diddle your expansions anyway for your new Stevie Wonder.

I use Strontium to minimize toxicity. I figure it's setting things up for the future. One day soon the powers-that-be are going to raise hell and shut a lot of people down over the toxins in glass. Look at what they just did shutting down the jewelry in CA for lead and smaller motorcycles/4 wheelers (kid size) for tiny amounts of lead in places like valve stems and control levers????? Sheeeesh. Wait until they figure how much lead there is in import color. OK, enough soapbox oratory....now back to our regularly scheduled programming.

Here's what I've been melting in pounds:

Sand - 97.5
Soda Ash - 36
Hydrated Lime - 5.5
Strontium Carb. - 9
Zinc Oxide - 3
Borax 5 mol - 9
Alumina Hydrate - 4.5
Lithium Carb. - 1
Potassium Nitrate - 1
Sodium Tripolyphosphate - 6.5

In percentage:

SiO2 - 65.32
Na2O - 17.32
K2O - .32
LiO - .27
CaO - 2.82
SrO - 4.25
ZnO - 2.02
B2O3 - 2.95
Al2O3 - 2.18
P2O5 - 2.54

I did some melts that were 4% ZnO and 4.25% SrO with no calcium at all, other ingredients the same and they worked fine also. A little nicer working and a little brighter when clear before the strike or if not striking the glass all the way.

These formulas will pull from the pot clear and strike on cooling or when touched with something cold. Lot's of nice veiled effects possible with repeated gathers and strike control via temp.

This was designed to melt quickly at a low temp. (2100F)and it does that quite nicely. You could probably lose some/all of the Borax and recalculate if you wanted less Sodium, less furnace attack and higher melt temps. I haven't tried it. I suspect that with the modifiers adjusted to these lower specs that borax and lithium may not be quite as necessary for knocking down the Phosphorous as we all thought in the past. That's theory, I haven't tried it.

Even getting close to 18% alkalai there's enough Alumina to satisfy Nick Labino's old rule of total alkalai percentage divided by 8 being the minimum percent for Alumina. Some clear unstruck samples of these phosphates have been lying around outside in the Florida humidity for a while with no deterioration. Nick was the man...that rule works!

I was using a coffee grinder in early experiments with pelletized superphosphate fertilizer and they do indeed reduce things nicely.

Reading between the lines in that old patent, the Calcium and Zinc levels are the most critical thing.

If you wanted to use bone ash it just sneaks in under the allowed percentage of Calcium so your batch would want no other Calcium besides what's in the bone ash. This assumes you use the same percentage or less of phosphorous via the bone ash as up above. Thought I would mention that in case you've been eyeing grandma's ashes as a glass additive.

One more plug for Strontium. It's supposed to be the closest thing out there to lead in terms of appearance and behavior in glass...without the toxic downside. That was Mr. Volf's opinion anyway. There's some interesting info on this in "Chemical Approach to Glass" under the Strontium section. I think I should have a "stimulus" check from the Strontium producers by now, no?

Pete VanderLaan
04-12-2009, 07:36 AM
What form were you introducing the strontium in? And... what did it cost in bag quantities? Are there 50lb bags of strontium (Carb, 90 , ) what? Can we make glowbars from it?

And did you ever get a L.E.C. on the stuff. I willl target it to a 96. I don't think I have strontium in the spreadsheet.

And with 150lbs of the tripolyphosphates coming this week and our soil not needing phosphates at all, I'm committed to the polyphosphates for a while at least. I have plenty of barium. And I have got to get my copy of Volf back from Mark Peiser.

Dave Bross
04-12-2009, 09:18 PM
Strontium Carbonate, $1.75 a pound in a 50# bag. If all else fails all pottery supplies will have it. This isn't going to help in New England but I got it from Davens Pottery Supply in Atlanta for less than what the major chemical supplier wanted.

About the glowbars, this stuff is no relation to the radioactive Strontium 90 and not useable as a silicon carbide heating element so I think that covers both possible answers there?

In my E&T spreadsheet I'm using a factor of 1.38 for Strontium to calculate expansion. I had forgotten that it's nearly identical expansion to Barium at 1.40 so there's an easy substitution.
That batch I wrote out will come in at 96 in my tiny melter but will probably be off a good bit in a melter with power.

Here's some from Volf from the Strontium chapter:

"Because of its identical charge and similar effective radius, r1, Sr is sometimes compared with Pb, despite the differing electron configurations of the two elements. The latter are reflected in the different polarizabilities of the two elements and their properties in glass. Pb as an ion of a B-subgroup element with a high atomic weight is twice as polarizable as Sr. In contrast to Pb, Sr with its lower atomic refractivity neither decreases the surface tension nor concentrates in the glass surface. Electrostatic charges therefore are not created on the surface of Sr glass by friction, as is the case with lead glasses. The closeness of the radii of Sr and Pb is responsible for the possibility of replacing considerable amounts of lead oxide in glasses containing 30% of PbO; in this way, Partridge reduced the PbO content from 30 to 20% by introducing 2-15% wt.-% of SrO."

There's more about how Strontium fines/melts better because it fluxes at a lower temp. and puts less gas into the melt than Barium.

Reading between the lines elsewhere in that chapter, Strontium does somethiing odd at over 15%, no particular details there as to what.

Another important point from Volf, Strontium glasses must have alumina in them or they will be less water resistant than calcium glasses.

Pete VanderLaan
04-13-2009, 05:19 AM
Thanks, that will save me a bunch of time finding the playing field. I was joking about the strontium 90 stuff. That is really interesting. I called Peiser last night and told him he has to copy Volf and send it back to me.

Dave Bross
04-13-2009, 07:44 AM
My pleasure.

One other "playing field" observation. Phosphates will be a slightly different E&T number than the usual number that works for a particular furnace for other glasses. Not so far off it wouldn't fit, given fairly liberal guidelines for a match...or at least it's that way it is here, which, of course, may not be an issue in someone elses equipment/location.

Rollin has been melting these for a little while so he may have some observations too.