color melt question

[Mitcheal Veenstra]

After many long years of saying I was going to start melting color in my own shop I finally got off my duff and reworked my old electric color pot and fired it up to start my journey of discovery with cullet colors for now.

I'm on my third attempt at color, the first was easily successful, the second successful in some ways, not so much in others(but not bad)..

the third, was odd.. and I think I know what happened but wanted to run it past the collective here. We decided to swap out pots and try the simple potassium dichromate green. The recipe that I pulled form the archives here calls for 30 grams of dichromate with 7 pounds of fritted clear. My base glass is spectrum 2.0 premium. I fritted the glass into a bucket of ice water, so I got nice small fines/frit. I used damp frit and made sure that the frit was well coated with the dichromate, well mixed is half melted....

After I had loaded the pot and let it run overnight, holding at 2100F (what I typically work spectrum 2.0 at in my shop) I checked the pot. There was a kind of dark haze in the middle of the top of the melt and the bubbles I expected as it was fining.. I went to poke the melt with a punty and the dark haze 'splashed' under the punty. Two thoughts went through my head, one was it's 2100 degrees, that's not water.. the second was, what the heck was it?!

I would assume it's some of the potassium dichromate that's puddled up on the surface after not fluxing into the cullet. I stirred the melt, pulled a color sample and found it was yes streaky and black specs and very green... I stirred again and then let it fine out for another 12 hours.

That night we pulled glass from the pot. The first piece was the worse, black specs, more bubbles than I liked.. The second looked pretty good, still a few black specs, but not nearly the size of the first piece. The third looked pretty clean. I know that the black specs is unmelted chrome that failed to flux in because chrome is notoriously hard to get into solution, especially with a commercial cullet.

now, to my question, was I correct? Was that puddle dichromate? How do I avoid that in the future, should I cut back the amount added to the cullet, melt hotter, or something else? Because the glass at the bottom of the pot seemed cleaner, I think perhaps the right answer is to cut back on the dichromate on a future melt, but I wanted to see what was your suggestions.

Thanks!

[Pete VanderLaan]

I don't think 30 grams in 7 lbs to be overkill. You could try 25. Chromes are really difficult to melt though and the dichromate really needs to be crushed before you try to add it. Adding to cullet is the hardest way to go and candidly 2100F is way too cold for that. What you see as black flecks are corundum stones and are the result of a cold melt with coarse dichromate. It's quite easy to crush in a mortar and pestle. When I teach that junk, I encourage students to give the dichromate a bath in lithium carb when crushing , but that's in a batch glass, not a cullet. Lithium is a powerful flux and it's not hard to throw the expansion all to hell. In that light, much as I hate the stuff, you could do the same thing in a bit of borax since it won't mess with the expansion much. The 20 mule team stuff at the grocery store will do.
Good Luck.

[Mitcheal Veenstra]

Peter, grind as in with a stone mortar and pestle? so the corundum stones come out of the dichromate once the chrome is fluxed in? I realize they are a crystalline form of aluminum oxide.

I thought lithium, but I wasn't sure if I could even use it with cullet, and I'd read also that it shifts the expansion quite a bit. I didn't really think about using Borax though it's a flux as well. When you say a bit, what scale of bit are you thinking?

I know coloring cullet is pretty basic and it will never be as nice as batch, but I had the electric color pot so I figured I'd start there. I've several books coming, Weyl, Scholes, and a copy of Courter's print of glass formulas. Volf is on my horizon and there's the Schmidt book I'd like to find that Cider Press put out a long time ago.. I'm starting from ground zero, or maybe a few inches above it with the craftweb archives and it's being an education. But it's a really neat feeling to open the door and pull hot color on a pipe or punty.

there's a gas color pot furnace in my future, I can see it also on the horizon...

Thanks for the advice and information. I appreciate it.

[Kenny Pieper]

Mitcheal I think you are going to have a hard time getting chrome to desolve well with cullet. It is quite refractory and I have found it needs lots of heat even when ground well and used in batch.

[Pete VanderLaan]

Kenny is saying it in a much less consoling way but it's too true. . When I melt chromes, I shoot at 2400F. 2100 ain't gonna cut it. If you really want a hopeless situation, use chrome oxide. It never works for anyone.
A mortar and pestle is nice. You can crush it between two bricks if you don't have the grinding tools but grinding is critical. . I used pieces of flat glass to do that, but really you need more heat and cullet is a tough way to go after green.

Also keep in mind Chrome is carcinogenic. Bullseye is still not allowed to melt chromes by Oregon. I also think that's an overreaction. Handle it with respect.
What will really help you out initially is to go in the Dave Bross method and mix very small quantities in a plastic bucket. It works fine. My first mixes, I did by hand and when I taught in Hawaii, I rolled a barrel down a hill into Claude Horan's Volvo. That was 1973. I can give you a formula that will fit your nuggets as a clear base. I think there's a 96 in the archives here that I wrote that is simple and doesn't use any exotic stuff. If it isn't here, PM me and I'll dig one out.

Good for you by the way. You will NEVER regret it. Especially after you see how much money it saves and how much faster you work.

[Pete VanderLaan]

If Joe Piefer still has any copies of the Thuringen Helmut Schmidt material from Thuringen in 1925 that Fynn Lyngaard translated, BUY IT. The formulations all need to be adjusted and there's some serious typos but it's a jewel.

The good books have the life of a mayfly. We're trying to get pumped up about doing the book we've talked about over the last 15 years. I want to more write a "field guide to doing this" since no one ever talks about process and process is damn important.Once the comics are sold. Get Helmer's secret batch book from Joe too. That's remarkable. Just a stupid title. At Corning, you can spend days in the library like I tend to and it too is a goldmine, perhaps for a few years down the road. Someone else needs to carry this torch pretty soon.

[Mitcheal Veenstra]

I've got Helmer's on order and I'm looking for a copy of Fynn Lyngaard's book, I've seen that one in a cider press edition form 1998 but I don't know where else to find it other than to luck into it.

I've been talking to as many of the folks who started back in the 70's as I can find to mine them for old cullet color recipes they may be willing to share with me and others with full attribution. I'm hoping to debug them as best I can with a modern cullet so that others can use them as well. I hate seeing this kind of stuff disappear. I collected an emerald green recipe recently that's looking good so far, we'll see what it looks like blown out in a few days.

I'll see if I can find the batch recipe for 96 in the archives, I'd like to try it at some point. I'm mixing small 7 pound color melts that will fit in my silly little single electric color pot and buckets for manual mixing are what we do...

I had another question that I've tried that didn't work in my second melt. My first melt was a simple cobalt blue.. I figured I'd start with something that couldn't fail. 3 grams cobalt oxide to 7 pounds clear and we had a beautiful blue. We had a suggestion of a change to make a 'flash' blue with the addition of silver nitrate to the next batch of blue and I was lead to believe it would reduce. I was skeptical but tried it anyway (I'll have to look at my notes to see how much silver nitrate I added, it wasn't much) and all we got was our first blue with some dark streaks that I assume is the silver nitrate... didn't matter how we reduced with the fluffy, still blue so I shelved that recipe until I could get books in and do some reading, but thought I'd ask here. I may be not understand what was meant by 'flash' blue..

[Pete VanderLaan]

I was under the impression that Joe bought the last 30 copies of Lyngaard's translation from his widow about three years ago. I handled that book some 10 years back and sold several hundred copies of it, so, it's out there somewhere.

Use three grams silver nitrate in the 7lb's of cullet. It will strike on the punty. Sooner if you reduce your gloryhole. If you want it to plate out, use more silver.

In cullet? formulas aren't so critical. It's all basic chromophores. and indeed is limited. Cobalt, copper, nickel, iron , silver work best. Stay under .1 percent. Manganese at a higher % stay out of reduction. Hodkin and Cousen had some formulas in it that worked OK.
Just buy a bag of SP87 color base and remember to mix it. It will give you a solid batch glass to work with. Then you can use cadmium, selenium and fluorine.

[Josh Bernbaum]

30g of dichromate in 7lbs seems like it would be super dense, but maybe okay if you're just using for first gathers that you'll case. In my experience, I have been doing okay with not seeing any corundum or streaks after a 2300 top end temp. but I am doing batch in a gas furnace, and the lithium bath and the finely ground dichromate from Peter's recommendations. I've heard from a couple of the boro guys that they have had trouble as well getting the chrome into solution with their cullet and their temps which are not nearly high enough in their underpowered kilns in my opinion. Gas furnace and batching process really removes most limitations that you'll see with electric and cullet, unless you're sticking to real basics like cobalt, copper or iron.

[Will Robertson]

Originally Posted by Pete VanderLaan

Someone else needs to carry this torch pretty soon.

Yup - the knowledge has skipped a generation and we're struggling to hold on to it in the face on increased environmental regulation and the secrecy that plagued the glass industry in the past (and still does plague some parts today).

Originally Posted by Pete VanderLaan

I want to more write a "field guide to doing this" since no one ever talks about process and process is damn important.

Definitely!

Re. dichromate - dichromate is carcinogenic so handle it very carefully and take very strict precautions to avoid dust or vapour being inhaled or escaping in your emissions.

There is a secret trick in the glass industry - if you want to avoid using dichromate you can use a finely ground mineral known as "chrome flour" which is Fe Cr O in variable proportions - the iron content makes it much easier to dissolve in most glasses than chromium oxide.

As Pete says, the lump formation in this glass is often due to the temperature being too low. Another cause can be the glass not being oxidizing enough - that will increase the risk of the chromium reducing from soluble Cr(VI) to almost-insoluble chromium oxide.

Have a go at making the glass batch more oxidizing - also check that conditions in your furnace are not reducing - that would result in faster oxygen loss from the surface of the melt - also consider using it more quickly - glasses often loose oxygen to the air if they're left molten for too long and that badly affects colours which rely on oxidizing conditions.

I make leaf-green glass and aventurine from recycled soda-lime (Pete will vomit but it was all I had at the time) and borosilicate glass but I always use it fast.

[Will Robertson]

Just replied above then read this - I need to learn to use the "More replies below current depth..." button more...

Yup - there's a fair amount of secrecy surrounding how to get chromium into glass and a lot of disinformation out there (on purpose I suspect) - I can understand some of the boro folks having big problems - it took me a few months to crack it (details in post above).