color base formula

[Bradley Howes]

Hello All,

I recently decided to undertake the challenge of developing a color base recipe. Looking at a couple recipes given to me from other people over the years and having worked with them, I pulled what I think are the desirable qualities from them and came up with my own. Here it is in oxide%:
SiO2: 74.00
Na2O: 12.00
K2O: 3.00
CaO: 7.75
Al2O3: 2.00
Li2O: .75
BaO: .50

E&T: 90.53

In weight%, it looks like this:
Silica: 60.16
Soda: 17.01
Potash: 2.28
Lime: 8.56
Custer: 9.87
Lith Carb: 1.58
Bar Carb: .56
note: these are based off the analysis of each ingredient, for example, the silica my school sells is 94% SiO2, 3% CaO, .1% MgO, .1% Fe2O3, and 3% LOI (lost on ignition).

Any thoughts on this before I buy enough to use up a 50lb bag of silica? I saw this was pretty damn close to the Spruce Pine 87 that was reported in Woolley's 1999 book, Glass Technology for the Studio. For working on my own color base, are there any flaws some of the more experienced people might see?

[Nick Delmatto]

What's Custer?

[Josh Bernbaum]

Originally Posted by Nick Delmatto

What's Custer?

I think it's feldspar. It says Custer on my bags of it anyway. Brand name?

[Bradley Howes]

No brand name to the Custer so far as I’ve seen. Here’s the breakdown for it.
SiO2 69.00
Al2O3 17.35
Na2O 2.91
K2O 10.28
CaO .30
Fe2O3 .12
LOI .04

[Pete VanderLaan]

Custer is a brand name for a feldspar Which if I recall correctly contains about 68% silica, 16% sodium carb and about 17% alumina. Plus or minus on everything and I haven't looked up the primary in custer since I don't use it. I use minispar, nice and clean.

I keep flashing back to your numbers and they are confusing. The first group have all "oxides supplied", the second is the actual poundage of chemicals in the goop? When you say Weight% is that the number of pounds of each material going in the mixer?

What I see is a glass really high in sodium which is going to make life hell in maturing colors. I would drop the sodium and increase the potassium carb to at least 3 percent , maybe more. I would get rid of the lithium as unaffordable and replace it with fluorspar if what you intend is to try to get it to melt at a lower temp. Currently, you're going to pay about $60.00 per 100 on the lithium and that gets old, pushing your batch costs well over $2.00 lb.

Try to reduce the calcium content by a lot. It's such a junk stabilizer Zinc or magnesium will give you far better color development. Since I don't see the custerspar represented by weight I can see soda coming from the spar and if one applies Labino's constant, it needs a bit more alumina in it. perhaps 2.2%.But it's hard to tell quite what you are doing. I could view that and say the alumina needs to be under 2.0 as well.

The school silica is a kind of WTF material. It's such a crap purity. Ideally you want a silica at %99.98 Si02 with just trace iron. I've never seen a silica sold at those percents and I have to question the numbers. If it's really the case, go out in town to a local pottery and buy a decent one. I have no idea why the stuff you have is so laden with impurities. What exactly has volition loss in a silica?

I think you're off to a good start here. I also think it needs reworking. When the color base was initially formed, I proposed to Tom a very simple way of doing it which was what he brought to the 2004 color class ( the very first one). All it did was to remove the antimony and the potassium or sodium nitrate from the original formula to render it a neutral glass, neither oxidizing or reducingbut replacing that nitrate with a carb or an OH.. It is a PIA to fine out however. We did hit a copper ruby with it on our first attempt which even surprised me. Not a great one but acceptable and we were melting at 7,000 feet.

So, I'm certain I'm misinterpreting some of your material here. Perhaps you could clarify some stuff. I may be coming over to Alfred in the coming months to look at some original texts in the Scholes library. Perhaps we could have lunch? Is LeCourse your instructor, or who? I had thought Alfred offered a BA degree in this but not more. Is that incorrect?

[Pete VanderLaan]

So, I have the alkaline supply backwards. The good part of that is it's at least a potash spar. It's just known as Custerspar, period. Again Minispar is cleaner stuff. There was one named Kona F4 but the plant burned to the ground.

If the 74% is the actual percentage of silica, that's an awful lot of silica. It could slide back to 70% and not hurt a thing. It would be quite durable though. Given the total alkaline supply at a bit under 16%, that's a fast setting rock hard glass. Hig calcium would make it runny. Further, if the intent is to match SP87 as a working fit, the expansion in E&T needs to get up to about 93.4. That's not saying 93.4 is what it really is but it gets you a ton closer in terms of mismatch.

[Bradley Howes]

The weight% would total 100 of whatever unit you were weighing with, but yes, it is what would go into the mixer. I built the recipe from the oxide% and then worked back to the material weights. This totaled ~117 so I just divided everything to scale it to 100.

The lower melting is what I liked the little bit of lithium for. I think I also remember hearing it makes for a pretty nice glass to work with. Does the fluorine from that just burn away leaving CaO in the melt?

For the alkali to alumina, I believe Nick's rule is an 8:1 ratio? That'd lead me to 1.96 for the alumina so I rounded it up to 2, not a lot extra but probably better to round up than down with this.

I went with the school silica because their other source of silica is labeled as Sand, 100% SiO2. But the mesh is pretty coarse, maybe around 120 or so. With the larger particle size, that would lead to longer/hotter melting. I think the LOI you ask about in the silica could be from the CaO and MgO as carbonates.

If you're ever around in town I'd love to meet up. LaCourse is still around and I've worked with him a couple times over the years. My advisor though is Angus Powers since I'm in the art school. The school of engineering offers bachelor, masters, and doctorate in glass engineering. A couple of my friends are writing a paper on aventurines which I'm coauthor on for some help I've given them. One of them, Charlie Bellows, is a PhD candidate and has told me from everything I've already taught myself from working with glass and reading lots of books on the topic, I already have a better knowledge than the students who actually graduate with a bachelors in glass engineering. A rather sad state of affairs for the school if thats true.

[Josh Bernbaum]

Originally Posted by Pete VanderLaan

I haven't looked up the primary in custer since I don't use it. I use minispar, nice and clean.

I found something online called "Minspar 200" which says it's a replacement for Kona F4. Is this what you've used instead of feldspar? If so, do you think it would differ much on expansion or oxide-percentage calculation if plugged into a spreadsheet in place of feldspar?

[Pete VanderLaan]

It's spot onand needs no adjusting. Nicer stuff.

Bradley,
I had written to the school saying I wanted to come over and addressed it to the co ordinator that shows up in the website. I got a response from her saying she would pass the info along and then the librarian followed up saying I could access the library but beyond that, not a peep.

Really, go out to a local pottery supply and get a decent silica. It should be finer than 140 mesh which leaves you at 200 mesh. 325 mesh is overkill. Do screen it. I haven't changed my opinion of sodium and calcium being junk . It does depend on what you are trying to make. That "little bit" of lithium is really expensive.

Always remember that in order to make a good color base, it needs to be a good clear base first.

If you can afford that much lithium, more power to you but I would be inclined to the fluorspar. It will assist in the melt process and have little effect on the final outcome. Potassium is going to make the glass fatter, but I like that. Since this is a color base, I don't think you would see that characteristic being an issue if it's cased in a nice clear.

Nice clear.. fast becoming an issue all by itself.

[Pete VanderLaan]

Originally Posted by Josh Bernbaum

I found something online called "Minspar 200" which says it's a replacement for Kona F4. Is this what you've used instead of feldspar? If so, do you think it would differ much on expansion or oxide-percentage calculation if plugged into a spreadsheet in place of feldspar?

****
Minispar is a feldspar so "instead" is misleading. It's mined in N. Carolina locally but it's sold all over the place. It did replace the Kona F4. It fits perfectly. It's very clean.

[Josh Bernbaum]

Not to drag this thread too far into the weeds, but what are the recommendations for what size sieve/screen to use for a feldspar (Custer or Minspar)? I just mixed up a batch yesterday where I used the Custer for the first time in years (been using alumina hydrate instead with the phosphates). I thought I'd remembered reading here that feldspar needed to be screened with a 140 mesh, so I bought a nice stainless one from Gilson Co. but in trying to use it for the first time it was painful to get the stuff to go through that fine of a mesh. I gave up after just getting a few grams to go through the thing. So I used the next-larger mesh I had which is a 20 mesh kitchen-thingy which probably didn't do much. What are others using for screening feldspars?

[Bradley Howes]

Originally Posted by Pete VanderLaan

Potassium is going to make the glass fatter, but I like that. Since this is a color base, I don't think you would see that characteristic being an issue if it's cased in a nice clear.

What do you mean by it being fatter?

[Pete VanderLaan]

I use about a 60 mesh screen and yes, it's slow , but it makes the glass a bunch nicer. It doesn't take all that long to screen enough for 100lbs of batch.

For most of my stuff I use the screen in rabbit hutch wire which is quite coarse. The disadvantage to the fine materials is that they will clump up. It's an ounce of prevention vs a pound of cure. Just stuff I've experienced that works well.

Fat? Well. SP is a glass I view as "runny" because of all that calcium and sodium combined. The downside is the devitrification and lack of luster. Mine is more viscous at 2140F and it doesn't just hit the floor. If you want mine more runny, you have to turn it up 15F and you'll unlikely notice any difference.

Ideally glass should be engineered to fit what the manufacturer needs in the glass. It's not hard to do, but it shrinks the qualified customer base. That's business.

Don't make castings glasses try to fit stemware production. You tool your studio for what you make. Glass is not an Isotoner glove.

[Eric Trulson]

Pete gave sort of a glancing blow to the distinction between a "fat"and "runny" glass, but I think it's worth expanding on that a bit, because I remember having a really hard time finding any information on those terms as well (I got into the glass world in a largely self-taught manner and relied entirely on resources online for the first few years). The main difference, as he says, is the change in viscosity - a fat glass will generally be more viscous and move less than a runny one at the same temperature. A fatter glass will also be less responsive to temperature changes than a runny one (or framed differently, fatter glasses have a wider temperature range where the glass is in the interim state between entirely liquid or solid)

You can see that difference in responsiveness to temperature changes best in cases where you're tooling the surface of a glass with metal, or heating it with a torch to try and melt in a surface imperfection and round the shape back out. A runny glass will allow surface tension to take over and let heat change its surface texture more easily than a fat one, whereas the fat one won't be as responsive to temperature changes and will tend to hold onto tooling marks more. Runny glasses will collapse more easily if blown thin and then reheated.

Fat glasses are great for thicker work like the scandinavians make, or for some processes like spin casting where you're using mostly heat and a non-contact force to move the glass. Runny glasses tend to be best for thinner work like italian stemware, or for production mold-blowing when you want a glass to quickly go from having quite a lot of heat and movement to chilling up and holding its shape.

Every assertion above is just my opinion based on my own experience and may be wrong. Like everything else in glass, it's kind of hard to tease out the exact cause for a glasses behavior, since formulation and melt process and temperature and thickness and timing and ambient conditions all overlap to influence the actual changes in behavior that we can see and feel.

[Pete VanderLaan]

In the beta test observations going on now, the glass was noted to have a very nice long viscosity curve. That means you can work it for a good amount of time.

[Mark Armstrong]

This is the custer feldspar I have used. These are the percent breakdowns and as seen by the label can vary from year to year. Same supplier of feldspar I'm told.


Custer Label.pdf

[Pete VanderLaan]

5KG? How cute! You can carry it home in your purse!

Thanks Mark.

[Bradley Howes]

Pete, I just spent a fair bit of time trolling through the thread from Feb, "Source for MgO in Batch". One of the tangents makes me wonder about the fluorspar you recommend adding for my batch. I assume it improves melting by lowering the viscosity due to the fluorine atoms replacing the bridging oxygens, and thus terminating the matrix, between the silica tetrahedra. However, without completely removing the calcium, is there then the possibility of crystallization from the presence of Ca and Al? I figure then it comes down to how much CaF2 gets added. Do you have a recommendation? Also, I see a lot of glasses with zinc. I originally first encountered zinc being described as an opacifier in calcedony. Now I see it cropping up everywhere in clear, SP color base, and color formulas. Part of my later mentioned HW includes looking at zinc but I'm wondering if you have some thoughts to give.

A note to Mark Carmen, thank god I beat my head into, and then through, a wall to learn my mole conversions. Going by atom quantities makes everything make so much more sense.

I'm doing some research right now into all these different batch ingredients, the different alkalies and earths, zinc, fixers/stabilizers, etc., before I come back with a revised base. Of course, nothing beats melting and seeing it, but hopefully I can save some time and money on non starters by doing some HW first.

Pete, if you're interested in swinging by Scholes, let's chat. I'm friends with John Hosford who's the art librarian. I don't often play middle man, but if you don't mind, I could make an exception in this case .

[Pete VanderLaan]

When I teach what little I know, a main thrust is to try to stick to what your eyes see, your nose smells and your hands manipulate. It may sound dumb, but I can't do the things Weyl talks about identifying carbon dioxide gas bubbles in a glass or the viscosity calculations of a certain glass. I don't have the tools. The tools I do have are testing mechanisms. The ring test, the Hagy seal and the dilatometer. I own all those processes and use them all. When I'm testing, nothing is ever drop dead conclusive, but I do want all of the tests pointing in the same direction.
So, in that spirit, I'll take a shot at your questions and can answer the first categorically regarding "disruption of the bridges made by oxygen disrupting the matrix."

I have no idea.

What I have experienced is that a small amount of calcium fluoride will aid in lowering the melt temps on most any glass and that if you stay under one percent there will not be a visible issue with crystals forming is an aluminum fluoride or a calcium fluoride crystal causing visible opalescence. Keep it low or electric furnaces will have major issues with it. Once you get up around three percent, you'll see the crystals and you will begin to see what we identify as Apatite stones in the glass. Avoiding them is where calcium carbonate is no longer used as the modifier and is replaced with strontium. It might be that using Magnesium would do the same, I've never tried it. I have been meaning to ask Mark why he used magnesium. I have never noticed things changing much if I went with a dolomitic spar and that usually had issues with tramp iron which I did not like.

Volf calls attention to the ratio of zinc to barium and I try to stick to that ratio as closely as I can in formulations these days. I first ran into Zinc in a glass way back with Fritz Dreisbach melting his "SZS" glass at pilchuck in the early '70's. It was the first time I had seen calcium replaced entirely with something else. The early days doing this were not loaded down with practical results at all. The SZS? It stood for "Sloppy Zinc Shit." It indeed was a very loose glass and was suited towards thin wares. This was a full fifteen years before SP87 came to be. While it may be true that zinc aids in opalesence, I've never experienced it having consequences. It just keeps getting passed down as something that happens. Initially I did put Zinc in my Calcedonia formulas since Dudley was doing it to his copper rubies and the two elements (Cu, Ag) were in the same intriguing column in the periodic table. I do run Ag glasses without zinc when I'm trying to make an oxidized silver-plating glass in reduction.

I don't do mole calculations. I should but I seem to get by quite well without them. In the few times they were pushed at me by Croucher, the formulas had mismatch issues, so I retreat to what I know.

I'm not going to make it to Alfred anytime soon. I'm swamped with two major projects right now that will become evident here in time. I do look forward to meeting you. I wish I could wave a magic wand on this but it's not to be. I do want permission from you to include this thread in a compilation on formulation issues. Email it to me please at that glass gu address I put up in one of the polling threads.

[Pete VanderLaan]

Just a few pertinent thoughts:

Given your refractories, a dissolution traffic cop comes around. Too much fluorine simply devours refractories. It's true of nitrates as well. Don't go past 1% lithium . Too much barium? Yes.

Balancing a glass formula is a testing process. I ran the PWV96 under some very critical watchful eyes from Croucher and Peiser and they were doing monday morningquarterback stuff. Tested six years.