View Full Version : Phosphates

Dave Bross
12-20-2016, 09:28 PM
Hi Rollin,

Nah, don't go with that recipe, it only works at random and it will quite often do the bad thing phosphates do... crystalising and leaving lumps and knots on the outside. It's also way off in viscosity so the expansion is a very odd number.

The trick to phosphates is in the modifiers. The phosphate glass is VERY particular about not going over certain percentages of modifiers and it absolutely must have some zinc.

No more than 3% calcium, 5%strontium, or 9% barium as modifiers. You can use some or all of them together, just not over that % for each one.

1 to 2% on the zinc. You can go up to 5% on the zinc but you don't need any more than 1% to stop the crystals from forming and it can make the glass a little grey at percentages over 3-4%.

Up to 5% phosphorous is supposed to work but I haven't needed to go over 2.5% to get the density I want. You can cool it and reheat it to strike it and it gets much denser white every time you strike it. Note: it also changes viscosity (stiffer) every time you strike it.
It comes out of the pot clear and then begins striking as it cools. You can get some terriffic effects from it in that partially struck state. Lower percentages of phosphorous are good for those cloudy effects too. Phosphorous doesn't begin to opacify the glass until you get up to around 1%

As you can see, due to the modifier percentages, the only way to get these is to mix 'em yourself as batch. Right up your alley.

Wait until you see the opaque colors you can make!!!!!!!!!! Oh-my-god... drop dead gorgeous! That cullet you saw was nothing compared to what you can make yourself.

While we're on the subject of phosphorous, try 0.5% phosphorous in your clear sometime. Brightens it up quite nicely.

All that basic info on the opaques came from a Corning patent. If you want to read more about it the US patent number is 4536480. I've done a number of melts following those guidelines and no trouble.

Get back to me if you have any more questions. Thanks again for that box of info you sent me a while back. That thing has been an absolute gold mine of info.




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.


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?


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.