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David Babb
04-25-2012, 01:10 PM
Greeting everyone,

I am a long-time lurker, but now I have a question for the group. :) Consider the images of Murano glass that I have attached. I love the very slight opacity and the ghostly bluish color that is produced by the preferential scattering in the glass. I'm not sure that this qualifies as lattimo, because the examples that you see referred to as lattimo are always completely opaque. The question is, how is this effect achieved?

The first (easiest) thing that I tried is to blow out opaline white very thin. Interestingly, you don't get the same effect, even with very small concentrations of white. You get a translucent white but not the color hue.

The next thought is that the effect is achieved by the growth of colloids in the glass. The effect is very similar to what one sees when using Gold Iris, which turns more opaque the longer you work it (as the crystals grow). This would also explain the light effects which strike me very much as an example Rayleigh scattering (the bluish tint caused by the very small crystals). My problem with this theory is that I have never seen a glass that strikes white. Although Gold Iris achieves the same opacity and light-scattering characteristics, it's still yellowish.

So, any thoughts? I should say that I blow glass out of the Material Science Department at Penn State. We would love to reproduce this glass, but without a sample to experiment on, my colleagues and I are stumped.

Regards,

David

Dave Bross
04-25-2012, 01:36 PM
I'm pretty sure it's a phosphate opal.

You can be sure by seeing if they transmit slightly red and reflect blue.

The trick is in the strike...as in not too much. From the look of those they possibly pulled two gathers and then got it made with no reheats. They were fast and good...if it had to be reheated the effect would be lost in a total white strike. The other possibilities could be a formula with very low % of phosphate or something to retard the strike. Haven't tried either myself. if one of those would work you wouldn't have to be fast or good.

The answers are here, buried in the posts. I'll give you a start then it's off to the search function for you if you want more info:

http://talk.craftweb.com/showthread.php?t=7024

Don't jump on the first things you see there. They're wrong. Read to the end.

I'll give you another clue. The opacification won't happen until somewhere around 1% phosphate. If it's a low percentage trick then you'll have a place to start. Where it happens in the neighborhood of 1% depends on what's in the batch otherwise...think expansion and viscosity.

David Babb
04-25-2012, 02:39 PM
Thanks, Dave. That sure sounds like a good place to start.

I should correct my original post in that "Rayleigh scattering" was not the correct term to use. Rather, I believe that these glasses are exhibiting the Tyndall Effect. See: http://en.wikipedia.org/wiki/Tyndall_effect There is a nice picture of a chunk of glass exhibiting the blue scattering and red transmittance that you describe.

David

Jordan Kube
04-25-2012, 03:54 PM
Some stuff by Patrick McCray you might like. You may have access to some of this stuff through school. I found it interesting. The girasole glasses are the ones you'd be interested in but all of the old formulas were full of lead and arsenic if I remember correctly. Dave's formulas are where you want to go.

“Glassmaking in Renaissance Italy: The Innovation of Venetian cristallo,” The Journal of Materials (Archaeotechnology series), May 1998: 14-19.

“The Unusual Optical Properties of Two Venetian Glasses,” with W.D. Kingery; Glass Technology, 1996, volume 37 (2), 57-68.

“The History and Technology of Renaissance Venetian Chalcedony Glass,” with Z. Osborne and W. D. Kingery; Rivista Della Stazione Sperimentale del Vetro, 1995, No. 5, pp.259-278.

“Venetian Girasole Glass: Investigation of its History and Properties,” with Z. Osborne and W. D. Kingery; 1995; Rivista Della Stazione Sperimentale del Vetro, 1995, No.1, pp.19-35.

“Technology of Venetian Girasole Glass,” with Z. A. Osborne and W. D. Kingery; 1995; Materials Issues in Art and Archaeology IV, MRS Press: Pittsburgh, PA, 201-211.

Pete VanderLaan
04-25-2012, 06:58 PM
If you are using Spruce Pine, Bob Held established 40 years ago that you could add up to 5% bone ash to a clear and still have it be compatible with a bone ash opal. The threads Dave has take not of the problems that occur when those glasses have too much calcium in them and their tendencies to form a variety of stones. Dave has I think made those opals work well. I have melted them and they were compatible with 96 expansion glasses.

When I did those melts recently, I didn't use real bone ash which is pretty elusive stuff but used Sodium Tri Phosphate instead which makes a good opal. Stones are always an issue in the learning curve but the glasses are beautiful.

You can't make it in a cullet.

Scott Novota
04-26-2012, 11:11 AM
Kugler Alabaster does the white misty opal strike from clear as it cools for the amusement of anyone watching.

Lawrence Duckworth
04-26-2012, 07:15 PM
Kugler Alabaster does the white misty opal strike from clear as it cools for the amusement of anyone watching.

what do you mean by "strike" can you explain?

thanks

Pete VanderLaan
04-27-2012, 11:59 AM
A strike is the formation of crystals within the non crystalline matrix of glass. I am not entirely convinced that what is in the picture is actually a phosphate opal .

Phosphates are emulsion glasses. You have a phosphate based glass intertwined with a silicate glass. On supercooling and then hard heating, the two glasses go in to what is called a phase separation and a bazillion tiny bubbles form in the matrix. What you see is opalescent and the glasses show both an opaline and a fired red color tone at the same time. They are difficult to melt, requiring a lot of patience, heat and proper formulation. Too much calcium in those glasses causes really ugly stone formations. They have a distinct advantage in maintaining similar viscosity curves to most clear glasses which the fluorine glasses ain't gonna do.

Fluorine based opals are different in their nature in that fluorine by itself doesn't cause opacity in glass at all, but in the presence of either aluminum or calcium, both calcium fluoride and aluminum fluoride crystals form. These glasses don't require phases separation to strike at all. You can go from a very faint milky glass to a fairly dense opal depending on percentage fluorine in the melt. It also takes sufficient AL or CA in the glass as well.

The last group of opals that concern studio glass people are really true enamel opaques. hey are the common glass used for cane work.They are made with about six percent arsenic oxide in a lead glass matrix. They form up a lead arsenate and are profoundly dense materials. They give off fumes that are quite toxic and are in my mind dangerous every where you use them. We know them commonly as the enamel whites and that sweet smell coming off of the powder is arsenic which will give you a fairly blinding headache if you use it without ventilation.

Jon Myers
04-28-2012, 04:01 PM
If you are using Spruce Pine, Bob Held established 40 years ago that you could add up to 5% bone ash to a clear and still have it be compatible with a bone ash opal. The threads Dave has take not of the problems that occur when those glasses have too much calcium in them and their tendencies to form a variety of stones. Dave has I think made those opals work well. I have melted them and they were compatible with 96 expansion glasses.

When I did those melts recently, I didn't use real bone ash which is pretty elusive stuff but used Sodium Tri Phosphate instead which makes a good opal. Stones are always an issue in the learning curve but the glasses are beautiful.

You can't make it in a cullet.
What is the stp/bone ash replacement ratio?

Pete VanderLaan
04-28-2012, 04:35 PM
I believe dave was starting to get a strike at about 2%. If you get up around five, it got quite dense but was throwing aphetite stones if the calcium content was too high.

Kenny Pieper
04-29-2012, 10:24 AM
What is the stp/bone ash replacement ratio?

Don't forget to account for the extra sodium,

Dave Bross
04-30-2012, 02:59 PM
And don't forget to keep your modifiers at 8% or above...without going over the given calcium percentage.

Bone ash has the problem of having enough calcium in it to be right on the line for too much calcium. The original bone ash used way back is hard to find now. It was the actual burnt bones instead of a chemical synthesis. it had a bit of fluorine in it which may be why it worked at percentages over what's suggested. I've found sources for guano (the original choice-of-phosphate-for opals...guanopal!) and phosphoric acid. I may experiment with these one day just to see. I found them at a fertilizer supplier on Ebay of all places. That would be kelp4less.

That's what the Strontium Carb. is for in my recipe...standing in for Barium in the Corning recipe because the Strontium is not a major toxin. The Strontium must also have alumina with it or it's not doing its modifier job durability-wise. Go with the Nick Labino "divide the percentage of alkalais by 8" rule to get your alumina percentage.

Striking happens around 1%. I was using a bit over 2% to end up with a fairly dense white (no, it's nowhere near what enamel white is) after multiple strikes. This was due to the many heats and cools required for what I was making.

Dave Bross
05-01-2012, 12:19 PM
One more thought...something came up in the current Manganese thread that reminded me to suggest you lower your standards.

Just kidding....sort of.

I never fully fined the phosphates. As soon as the big bubs were gone it was on the pipe and gone. The little bubs actually give a pretty effect. They do fine pretty quickly if you are particular that way.