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Louis Copper 04-12-2019 11:55 AM

Trouble kiln casting with billets made from Spruce
Hi everyone... I haven't posted in years, but I am in need of some wisdom, and decided to see what you thought.

I recently had the opportunity to make some of my own colors using SprucePine batch and various metal oxides in a high temperature kiln. At first I only made small test samples just to tweak the color. I made two colors that I really liked, a golden yellow and a blue, and invested in enough batch and chemicals to make perhaps 400 lbs of the two..

I had some time with a furnace, melted the batch, and then formed billets by ladling the glass into a frame on a marver table, and then putting them into an annealer. The billets were beeeuuuutiful.

Then I tried to kiln cast with this glass, using a flower pot as a reservoir, and flowing the glass into a plaster/silica mold. and it was a disaster... extreme veiling and devitrification, with a lot of bubble formation at the mold wall. The shapes were 10" diameter spheres, solid castings, so the top temp hold time and annealing time were necessarily quite long. Smaller castings with other glass in the same kiln run were clear. (glass specifically made for casting.) ((all molds were plaster/silica of similar mix))
It seems that I overlooked one TINY detail about this glass. I have now been informed that batch for blowing is formulated with a goal to make glass of even viscosity over a wide range of temperatures. Casting glass batch is formulated (I am told) to resist de-vit and flow at a lower temperature. And I clearly did not use it as intended....

So my questions are these...
What is a typical formula for a blowing glass batch?
And a casting glass batch?
Are the fundamental ratios of glass former, flux and stabilizer different in the two?
Or is one a kind of subset of the other, but with additional additives to provide the altered characteristics needed for casting?
Is there any feasible additive (or set of additives) that can be added to batch made for blowing (if possible, specifically Spruce Pine, since that is what I now have on hand) such that it will produce glass that is robust enough for casting?

The glass may not be a total loss, if I can find a threshold of heat work that it can tolerate that can still permit thin castings. So far all heat work tests, even as short as possible, show de-vit..

And one final request for input... Is the following analysis correct? Please correct/clarify it for me if you can.
blowing glass stays for days in a furnace, and resists cords and property changes over time, and yet does not perform well at the much lower temperature when it comes to components within the glass beginning to crystallize and cause devitrification. In hindsight I guess this is because at high temperature the heat is sufficient to keep everything molten and crystallization is not possible.. degradation happens over time by a slow loss of flux from the melt. When this same material is much lower in temperature it is in a temperature region where it's component elements can begin to crystalize out of the melt. Casting glass has added components that reduce the mobility of the materials that tend to crystallize so that the crystals do not form as readily and increases the complexity of the melt in a way that interferes with and slows the crystallization characteristic of de-vit.

Any thoughts would be helpful

Shawn Everette 04-12-2019 12:27 PM

I've never had great success with Spruce as a casting glass. You can do it, but devit is common, and it tends to look cordier.

To avoid devit in any situation you're going to want to crash the kiln after your high temp soak down to your anneal. I'm not sure what the devit temp is for Spruce, but most kiln glasses you're looking in the 1250-1350 range. You also might have more success with a pre filled mold and either spruing or topping off. If the devit temp is around your casting temp, the more it has to move, and the longer you leave it there, the more it will crystallize.

I also know that in some situations certain colors are going to be more prone to devit.

Pete VanderLaan 04-12-2019 01:46 PM

The questions surrounding what makes a good casting glass as opposed to a good blowing glass sound simple, but they're not.

Shawn is certainly on the money when he says SP87 is a lousy casting glass. It's quite true and making billets of it and then melting a second time is tweaking the nose of Buddha.

Modern Glass practices by Sam Scholes lays out the parameters for what different materials do to a glass and they are worth noting. In my color classes I try to stress that you can't make a good colored glass until you can make a good clear. I also think it's covered to a certain extent in Glassnotes.

If I wanted to make a casting glass that worked in the Anne Robinson process with the flower pots, I'd be looking at having materials that would lower the viscosity a good deal. There are the following to consider: Boron coming from borax, Fluorine from Fluorspar ( calcium fluoride) Lithium carb, and barium if you aren't doing lead.

It will need alumina in a ratio taking the total alkaline fluxes and dividing by eight, so, a 16% flux glass will need 2% alumina. You will need more than 16% though. Boron lowers viscosity but it also eats at your furnace. It is not a favored nation material in my toolbox. Fluorspar is powerful stuff and also attacks refractories as well as all electrical parts. Barium can be substituted in a glass for calcium or lead. It brightens the glass notably and helps reduce viscosity. At over 3% it will start to give you trouble. It really makes blue colors jump. I really like potassium in my glasses but they increase viscosity while increasing luster. It also really enriches color. I view Soda Ash and calcium as sort of trash materials. I use them but I don't lean on them beyond the basics. . Keep your silica content at 70% or so. Mine has a good deal of potassium in a nitrate form which helps with the melt. You need Antimony as well, not much but don't start without it.
My formula, available from Spruce Pine won't devitrify, has high luster but is a bit more viscous than SP87. It's the potassium that does that, so, it might take an increase in temps to flow. Beyond that, Gaffer makes great casting glasses as does Bullseye. Both are pricey. The colors from Gaffer are infinitely better.

That's a real nutshell. The subject takes much more time if you are picking at it. There's many things I've left out. I think a thorough search of the archives may surprise you. There's a lot in there from the last 18 years.

Eben Horton 04-13-2019 08:49 AM

Have you considered taking the billits and re melt them in a furnace and use the glass to ladle cast? Spruce pine does indeed struggle with kiln casting but it is great if you hot pour it into sand or graphite molds. My experience hot pouring it into plaster is limited, so I can’t comment on that.

Shawn Everette 04-13-2019 09:18 AM

You'd want to reinforce the plaster or use R&R, that much heat kills what strength the plaster has left. I've seen it done poorly, and a kiln blow out is not something you want to deal with. Then there is the trick of juggling a ladle into a kiln and getting a clean poor.

Louis Copper 04-13-2019 11:55 AM

Thanks and follow-up
Thanks Shawn, Pete and Eben…

It was great of you all to take time to comment, and I learned a lot overnight.
In a way I feel comforted (though the problem has no easy fix) just knowing I am not crazy and other people know what I’m talking about.

Shawn, as a little test, I cast pieces of these billets in a little test kiln I have… to form a little block about 1” x 2” x 2” in a plaster/silica mold.. I did it as fast as I could in the kiln, up and down, and crashed the kiln after 15 minutes for the chunks to form into the block.. All together, up and down, I’d say the glass was at 1250 – 1550F for about one hour. I still got devit, so kiln casting with the glass I formed already is probably not feasible.

The spheres were prefilled as much as possible, and the remaining glass was in the flower pot. There was a clear difference between the preloaded and the glass that flowed out of the pot, we could see the level on the casting, and the preloaded glass was better, but still massively crystallized.

I have had that experience with different Bullseye colors de-vitrifying to different degrees. (all surface de-vit though.) for now though, that difference is laughably small compared to the big de-vit happening due to the basic glass.

Pete.. I gather Buddha does not like his nose tweaked! He responded emphatically. Thanks for the reference book leads.
I read my Glassnotes during this project, but I found it to be at a ‘general information’ level. I will get my hands on the Modern Glass Practices book.

Thanks for the tip about lowering the viscosity… I am wondering if you could say more about why that was your immediate thought and a possible way to address my issue..
Your boron suggestion had occurred to me, but I was thinking it would be better to add a boron source that did not also add sodium… so I was going to buy boric acid instead of borax. Am I shooting myself in the foot somehow?
Similarly, if I add fluorspar, I simultaneously add both Ca and F… would the Ca portion need to be backed down to allow the addition, and would the effect of the F be greater on the flow properties than that caused by the Ca?
As far as Li goes, I thought one of the key differences between Spruce Pine and other blowing batch is the presence of lithia, and I was associating the very high level of de-vit with SprucePine batch with it. ( I was recently told that the Gaffer batch recipe that Spruce Pine also makes does not contain it, and works better for my type of casting process..) So I clearly have a misguided understanding of what the Li is actually doing or why more of it might be good.
The idea of changing batch with additives that eat up equipment pains my conscience, as the equipment we have access to is not ours, and we best not be changing that aspect of the batch very much. Does boron do immediate damage at the lowest levels? I certainly wouldn’t want to shorten my friends’ equipment life. F too? Ba is new to me as an ingredient, and I will look into that one.

Would you mind explaining the source of the rule of thumb you are using for the alumina being 1/8 the total flux? What is your understanding of the chemical reason that is helpful and the right amount? Perhaps it is an empirical finding based on deep experience, but I would be interested in your thoughts about why it works.

I found your insights about the materials and the addition amounts to be very helpful and interesting. As a newbie to glass formulation, it is the levels that are difficult to get a clear picture on… and seem to be the most closely guarded part of the ‘trade secret’ of commercial art glass. So thanks for sharing that.. It is good food for thought, and immediately puts more realistic bounds on sample tests, and saves a ton of effort.

Do you mean that you also have a formula batch that Spruce Pine makes and sells? (is the Gaffer formula yours?) Sorry if I am talking to the inventor without knowing you were involved! Would your formula work for kiln casting do you think?

Lastly, your point about searching the archives is well taken… I will try to do some homework searching the archives before I jump in with questions. It is so tempting to just ask, but if the questions have been tackled before I should give that a try first. I struggle sometimes to find effective key words, but the truth is I need to give the archive a whirl and see how it goes.

Eben, thanks for your thought about ladle casting. Ladle casting is awkward in the studio I have access to… I am told they have done some, but the kilns are not designed for it, and the furnace and kilns are quite far apart. I like the graphite mold idea, and have a couple shapes I want to pursue, so will look into it. People I have talked to about ladle casting into plaster/silica give very mixed reviews, which is not surprising considering the decomposition temp of the plaster.

Bottom line… thank you everyone for your input. I got a ton of value from the exchange.. Lou

Shawn Everette 04-13-2019 01:32 PM

One of the problems that you are going to run into is simply mass. The size of the objects that you are trying to make is going to cause a lot of heat retention, so crashing the kiln efficiently is going to be a problem. Considering that your test block had devit, then you're probably not going to have a great work around with the base spruce.

Pete would have better knowledge about casting batches. The glass is out there so there must be a batch recipe somewhere. I just thumbed through Hellmers' and didn't see any casting references.

You're probably going to experience some devit with the bullseye, especially if you are not using billets. The more surface area the more possibility for dirt and imperfections, and you increase the potential for the crystals to start forming.

It's properties are also really developed with fusing in mind as well, so it's missing the major characteristic of low viscosity at low temp. You'll have the same issues if you try to use anything from Spectrum/Oceanside.

I've personally not used it yet, but the Gaffer is the only readily available casting glass that I know of anymore that is in high demand. The production backlog may put the squeeze on you. Reichenbach is also available, but I've never seen anyone use it, and I personally think the gaffer pallet is nicer.

On an aside, if all your wanting is spheres, and you have access to a shop, why cast? I remember helping on a roughly 10" sphere at a Scotty D workshop a number of years back. It was a heavy bastard, 42lb if memory serves, but it can be done.

Pete VanderLaan 04-13-2019 01:51 PM

Shawn points out that the mass is going to change the kiln performance which if you go slow with SP87, voluntarily or not, you're going to see devit. While I know SP's formula, I'm not at liberty to divulge that at Tom's request. I di not write the gaffer formula at all but I do know that John made it to match Gaffer colors and there were no other considerations.

I have slumped large rings of my formula which I blew out , cut on a saw and then slumped and they slumped beautifully with no sign of devit at all. Slumps are handy because they push the glass to places where it may not want to go. It will tell you a lot about your glass. (Just nice high luster and it went down at about 1450F. I like it as my best work in formulation. SP does sell it but I think by the ton until there is sufficient demand that they could have some premixed.

The alumina flux ratio has been around a fair length of time. It was the creation of of Nick Labino, one of the early pioneers in studio formulas. It works very well as a ratio. Alumina is really important in avoiding devit but keeping the material out of the happy devit range keeps Buddha happy

I would council you to stay away from boric acid as an additive. I would say pretty much the same thing about borax. I'm not a fan. I do think the calcium fluoride (CaF) would be the place I would start. You can buy that from Universal Pigment. Not good if you're running electric. I'd go to lithium if that was the case and it's far more squirrely to deal with.

Try to solve one problem at a time with one change in process at a time. Too many and you will start to get really conflicted.

John Croucher 04-13-2019 06:13 PM

The provenance of this "rule" of 2% alumina oxide in the glass for maximum durability can be found at:

Lyle, A.K., Horak, W., and Sharp, D.E., The effect of alumina upon the chemical durability of sand-soda-lime glasses, J. Amer. Ceram. Soc. Vol 19, 142-147, 1936.

I think the problem of SP devitrifying when it is used in a lost wax casting situation is the high level of Li2O, notwithstanding the substantial amount of alumina in the batch which you might think would counteract it.

Louis Copper 04-14-2019 01:31 PM

follow follow up
Thanks for the follow-up guys.

Shawn, I get your point about mass; with the glass itself, the molds and the kiln furniture to support everything, the controller is often waiting on the mass to cool… and times are prolonged at bad places in the heat cycle. No matter what I do, prolonged time at temp has to be taken in stride by the glass I use.

So, unfortunately, the use of Spruce Pine blowing batch for casting probably goes in the ‘Lou’s Folly” category… and rather than try to make a pick-up out of a sedan, I should go find a pickup, or something framed for it.. If glass recipes were that robust, people wouldn’t have to fine-tune them for their particular use like they do.

Years ago when I was slumping and using mostly BE sheet, I saved every sheet scrap, because it was implied (or not explicitly denied) that all those scraps could be useful casting material. Just recently, though, I asked the direct question is the batch for their billets the same as the batch for their sheets…. They did not answer… so I am thinking they are indeed different. You can see differences when you cast with sheet scrap Vs billet, but it is easy to attribute the differences to the form and trapped air, higher surface. But there is also more veiling, and visible flow lines with some colors… i.e. de-vit, perhaps.

Spectrum 96 had casting rocks in the old days, and clear casting plate. The rocks were produced by Uroboros, and the plate by Spectrum. I actually had good results with the plate, and bought quite a lot of it. All that came to a halt when operations ceased in Portland. I am really thankful that Oceanside came along and bought Spectrum, but so far the consideration for casting glass has been on the back burner as they ramp up and catch up demand for sheet, frit, stringer, etc. The plate was actually continuous ribbon made, with the thickness adjusted to about ˝ inch. I am also thinking that they are doing some market review, trying to see if casting glass is worth doing… not sure though. The price point of Gaffer and Banas (Czech) and Bullseye seem to invite competition though. I pretty consistently found Spectrum more resistant to de-vit than Bullseye for casting, and I have had others tell me the same thing about fusing.

One of the reasons we tried to cast with Spruce Pine is that, if our sources of less expensive casting glass dried up, then it would be a way to keep working. Gaffer is lovely stuff, but if you make medium to big stuff, you can have hundreds of $ in one piece. Even with all our dinking around we made some beautiful SP colors for $3 a pound… so we had to try.

I get you about casting a sphere too.. blowing a bubble makes a lot more sense for round… but we planned further processing, cutting and polishing, as one part of a larger assembly, and solid was needed for that.

Pete; there are words to that effect on the GafferUSA website… that they developed the batch as a representative middle-of-the-road formula for their colors, reasoning the colors would thereby be useful to the most people. Casting was not on their minds or at least was not mentioned.

Spruce Pine helped me out when I wanted 100 lbs of their color base. It is a real hassle for them though, their weighing equipment is made, as you say, for large quantities, and 100 lbs is about the least they can measure out. And as 100 lbs is too big for UPS, they split it into 50 lb bags, some ingredients in one, some in the other… not normal. They have to bag it after weighing, not where they normally do which is after large scale blending, so that is a weird equipment arrangement and extra handling. They have to charge extra just because of all the labor to screw around like this. Tons of product actually requires less labor… one guy only at bagging for example… I would like someday to try your formula, but I think a more reasonable approach would be to wait for the next big batch order to come in, and then piggy back on it… So if you or someone wanted 3000 lbs, Spruce Pine could make 3300, and have 300 to sell to me. Right now I want to run some tests. You like/developed it for blowing and have had good slumping results as well, and it sounds good. I would like to try to carry on my color testing and casting with it as a new basis. What is really good too is the blowers we share space with might like it too, and then we could have a raw material useful to all.

Now that you say the name Labino, it rings a bell. And now that I am aware of Buddha’s feelings on the matter, I am a converted believer in keeping him happy too.

I heard your point about equipment corrosion with the boron, and perhaps problems difficult to unravel when it is present. Is it bad for other reasons? Our color and comparison tests so far have been in an electric kiln in small crucibles, so perhaps the CaF2 should be tried in the gas fired furnace later.

John, thanks for the reference about Alumina… I will request it.

Thank you all for sharing your experience. Now I have some ways forward I had not thought of, and am not dead in the water. Lou

Pete VanderLaan 04-14-2019 02:41 PM

SP color base has to be mixed well. That's A . B, is that it does not contain fining oxidizers making it easier to do colors with copper ans silver but it's a real downside.

I wrote my formula to accommodate tom on a secondary batch but I used better quality materials to do what I wanted.Subsequently it didn't cost less than SP87. My response is that I don't like making inferior products and I view SP87 as a limited glass. It does fine for blowing, not for casting. It is far lower luster but has a lower viscosity. I'm not in the position where I want to sell small quantities of my glass. Making my own work takes enough of my time.

There's no free lunch. If you want a glass with less viscosity, you need to engineer those results and the advantages will have disadvantages. I don't view Spectrum as a boon to the field given all the problems that came up as well. As to Oceanside, they have a long ways to go to inspire confidence.

If you indeed have mass issues in the kiln. Crash the kiln by opening the door until it hits 1100F.

I haven't read the material John refers to. If it was asserting 2%, that's a bit different than what Nick preached. I will continue to total the alkaline flux and divide by 8. Alumina is strange stuff. I have tech material from Vargin for enamels where the silica content is less than 50% and the alumina hovers around 11%

Shawn Everette 04-14-2019 03:52 PM

There is no getting around mass, but venting should help. You need to be somewhat careful about to to prevent thermal shock. Pete is right about venting to 1100, but let it bounce up and vent again, rinse, repeat. If you're using bullseye you may need to hold at 1200 to prevent "suckers", weird areas where the glass pulls away from the mold. They don't have a great explanation on why it occurs, but I have seen it.

The billets and plate should be relatively the same glass. They're both compatible, flow the same, and nothing in your cycle changes. The biggest difference is the color saturation. Transparent fusing glass is designed to be less dense than a blowing color because it isn't going to be stretch much and is usually used in a 3-6mm application. Casting glass is even less dense because most castings are going to be much thicker than that. Casting glass can have lower viscosity, but bullseye's nor specturm differentiates cycles between billet/rock or frit/plate. Both need to be fires 1500+ to get any kind of flow. There is even some gibberish I just read that was saying that bullseye's surface tension increased past 1550.

If you decide to stick with the flower pot method I highly suggest kilns that are top fired, and using the widest possible vessel you can find. You will get significantly faster melts.

If you are not using billets then you are always going to have some form of veiling. The plate and frit that you load into a kiln is going to have a much greater surface area than a billet, and more surface = more imperfections. It is inevitable that the broken glass remelting is going to trap those imperfections, and you are not going to get to a high enough temperature in a casting to have those fine out. Glass doesn't readily go back into solution unless you are at temps well above a casting.

Quality casting glass is always going to be more expensive. Purity has to be extremely high, shipping is pricey, and the demand is lower. I wouldn't hold out on Oceanside producing rocks anytime soon, and the verdict is still out on QC. Gaffer is going to give you a lower temp glass with a wide pallet. It's great that you want to make your own to be economical, but you're really searching for a different base formula than blowing or fusing.

The orb we made was solid glass. I suggest finding some football players and teaching them how to turn pipe.

John Croucher 04-14-2019 04:29 PM

Perhaps I shouldn't have said the "2% rule". The article by Lyle et al specifically says that "alumina exhibits the greatest improvement in chemical durability when the amount is 1/8th of the alkali content of the glass" As most studio glasses have alkali levels around 16-18% then 1/8th equates roughly to 2%.
The article does go on to make an important observation that a glass of 17% soda and 1.5% alumina has the same resistance to chemical attack as a glass with 16% soda and 0.5% alumina.

Pete VanderLaan 04-14-2019 05:55 PM

yet you would attribute the effects of Lithium at a .5% level as being a likely suspect in the tendency for SP 87 to devitrify in a fixed temp range. Many glasses will devitrify in cold spots in large furnaces as I've always understood it. If, as you and I looked at, a few weeks back that SP87 has in fact had variations and the one we poked at listed the Li content at 1%, not .5%, we're both clear about where Tom finally landed but I'm still looking for this feature of lithium, beyond the radius which would have such an effect.

I left lithium out of my formulation for Tom but that was based in the suspicion of price and availability of the stuff. Beyond those additives, I tried to stay true to Tom's formula where I could and decided instead to use nitrates to drive melt characteristics down. I can assure you, mine does not even hint at devit, slumps readily and doesn't attack a 90% alumina refractory. Pretty cool. I would be interested in your thoughts on the two glass differences as to Devit. You have both formulas.

Louis Copper 04-15-2019 01:09 PM

additional reply
Hello again. And thank you again for your continued conversation and thoughts about this.

You all have such a different angle on glass than I, and greater depth of understanding along the whole chain from formulation, to fusion, and then to use. I came to working already-made glass bought from a few limited suppliers and then forming it in a kiln. Our conversation exemplifies just how narrow a slice I have been focused on. (I have never really done blowing either, as it frightens me.)

I am not really a proponent of anyone’s glass and did not mean to sound like I was. I have found that some glasses are better suited to specific projects/techniques than others.. I have often wished that I could make my own color just so I could have more range of colors and densities… of course without realizing the full implications of doing my own formulation of batch… a whole art/science in itself.

You guys made a great point about crashing the kiln, and we just have to be wiser about scheduling so we can be there to do it… it is worth the trouble, and we have ignored a step we can take by assuming it not to be significant.

The Spruce Pine people really emphasized that I need to mix color base well, and thanks for that point. I will take extra care with it. I hear about the fining oxidizers, and I will see it I guess. I can add some Sb oxide if the fining is poor, but first thing is to check the color base alone. We are trying some commercial crucibles made with different formulations to see if any color is leached out of the crucible… in order to find the one most inert to color. the WER formula is made to reduce its sensitivity to colorant impurities.. iron mostly I gather… so we asked for the color base to have a more sensitive indicator.
I get your philosophy of spending more to get the better result, and am in camp with you on that. If I can pick a better batch to work with, I am happy to pay the higher price for it.
I did not take what you and John said as being different, just as different ways of referring to the same guideline/rule.

I have a take on suckers… my understanding is that a certain set of circumstances are needed for them to form. If you have a mold with a contiguous built-in reservoir, and the volume of glass is a bit excess, then when the mold is full there will be a residual level in the reservoir. The trouble arises when the top surface of that ‘casting pool’ in the reservoir solidifies before the interior glass. Because of thermal contraction, if the casting is contiguous with the reservoir, then as the whole mass cools, it must contract. If the casting pool is ‘frozen over’ and it cannot contract from there, it will pull in somewhere along the mold wall, usually at one of the warmest spots remaining in the assembly, as that is where movement is easiest.
This is exacerbated when the freeze over happens with a very large temperature difference between interior and frozen surface, as contraction is greater the greater the difference. So this is actually a reason not to dump the heat, if doing so causes your reservoir to become immobile.. Part of the reason for the recommendation to hold at 1200F is that will help reduce the difference in the two temperatures. Other people cover the reservoir with a fiber blanket as the heat is dumped so the reservoir glass doesn’t get way ahead of the rest, and the molds and furniture have a chance to cool more too. One of the things about the flower pot or any sort of separate reservoir is it is no longer a confined and contiguous piece of glass. Any way, I got this idea from one instructor I had a Corning, and it makes sense to me.. you can take it for what its worth… clearer than the BE explanation, for sure.

I see your logic about the price of casting glass. Oceanside is an unknown since they haven’t begun to work on it, and have a smaller color selection than BE in the recipes they purchased from Spectrum and have a lot of development to do to approach the color range of BE. And I think they inherited quality problems from Uroboros (who had de-vit issues) and Spectrum (who had stone issues). I hope they don’t stop production altogether though, if for no other reason than to keep BE thinking. If I had the money I would consider buying Gaffer only, and make more work and do less tests. Even as I say so, though, I know that the color tests I have done have taught me so much, that it was well worth it, despite some money down the tube for glass I can’t cast.

It never occurred to me you were talking about making a solid orb on a pipe… a symptom of my inexperience there!

thanks for the clarification of the rule, and for the article. I think you confirmed the common basis. I hope to read that for whatever background understanding I can glean from it.

I will be out of touch for an out of town trip, and wanted you to know I won’t have access while I’m away. Not that I’m expecting any kind of answer beyond what you have already given, but just so you know if I go silent for a while. You have been most helpful, and I have many things to ‘go forth’ with, and the thread can end now if it's time to move on. Thank you for sharing your knowledge.

Pete VanderLaan 04-15-2019 03:35 PM

I live in the land of no free lunch. We all wish glass could solve universal problems instead of being the universal solvent. Most people think of glass as having afixed expansion at a fixed temperature and it does but it keeps on expanding which is why it's impossible to fill a pot to the brim. What looks full at 2300F is not what it will look like at 2000F.

Anne Robinson is the key to your casting. You need to determine exactly how much glass you need to fill that mold, allowing just a bit at the top sprue. That can be done and Henry describes it wll in Glassnotes IV as a simple displacement issue. A lot of people use rice to get there.

Spruce Pine color base is simply SP87 without the antimony or the nitrates. In making many colors that rely on colloidal suspensions, getting oxygen out of the glass is key to success. Other issues do come up but that's a major hurdle. Tom took the color base to heart after taking my first color class over fifteen years back. It helps to count your oxygen in your glass and to do that you have to work from the raw chemistry.

I only advocate making your own glass these days so you know where your glass comes from but you have to always keep in mind what brought you to the swamp. Making the piece is the important thing. The tech part will try to hinder you and there are not many tools on the market that make this easy. You just get that appearance. It is never easy

I can't say about your crucibles, I don't know what they are. Assay crucibles have a place in the show as do high purity ones. Iron can be an influence in color but you have to be at a pretty fussy place to notice that.

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