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Old 04-10-2018, 12:52 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Glass Body Reduction

Could electrical current flowing through a crucible create reducing conditions in the glass body?
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Old 04-10-2018, 02:02 PM
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I can't evision what would be being reduced ie: having oxygen taken from it and by what?
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Old 04-10-2018, 02:35 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Well, the other half of the redox reaction is the acceptance of electrons. If the oxygen in the glass is overcome by electrons supplied through electricity (rather than a reducing chemical like black tin) than it would seem to push the redox balance towards reduction. The various chemicals in the glass body would then be susceptible to electron donation.
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Old 04-10-2018, 04:20 PM
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That is oxidation. The oxygen in the glass would have to be reduced to fit your scenario. I don't see that happening anywhere but in the atmospheric section of the furnace and the top 1/2 inch of the glass is affected by those conditions.

You aren't going to change a n oxidizing glass body unless you were to take the cullet, frit it and then remelt it. The melt initially would cause O2 loss in the melt and then fritted and remelted, there would be less O2 and subsequently a glass somewhat more receptive to the advantages that glass would have. Peet Robison used to make a ruby with copper using that approach but it's BTU expensive not to mention the time involved.

SP 87 color batch, which came into existence after Tom took the first color class if I recall correctly ( perhaps not) struck the red on the first attempt. It was not a great red but red it was. I didn't like his feldspar at the time. We could have refined that I think. Copper reds like a really clean lime source- hydrated kosher was the best.


When I look here at the books of melt attempts, it's really astounding how many there have been over the decades. Binders and binders, years and years.
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Old 04-10-2018, 06:20 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Thanks for the response. I'll fill in some details...

I have been working with the silver opal and moved on from the fritted cullet to your unoxidized batch from Jim at East Bay. I was having issues with the cullet in what appeared to me was over-reduction. It varies, but in one of the more frustrating scenarios, the glass starts to strike beautifully (purple, blue, green) only to be overcome by a dark amber color. The less black tin I added, the lighter the amber.

This continued with the batch. I added a full 28g black tin to the typical 10lb recipe and it was very dark. I finished the weekend with 10g black tin in 10 lbs and the glass worked for 3 pieces and then shut down/went amber. It was clear when gathered. I stirred (no yam) but it didn't improve. Good news is the batch is working great...no clumps, passed the ring test.

So far I have considered 3 reasons for the overreduction.
1) The black tin from previous melts is sticking around and contaminating my pot.

2) I live at elevation (6000ft) and my effective oxygen is lower and therefore my glass is more easily reduced.

3) The question I alluded to about electricity. I received a nice jolt while gathering from the color pot. I reached down to grab my shears with the pipe in the glass which shorted right through me. I've felt a little buzz in the past but this was a bit more. I think an element has popped out of a groove and may be contacting the pot directly. Got me wondering about how applied electricity might affect the glass chemistry. My color pot is silica and electrodes have been made from this material. It's not inconceivable that this is providing free electrons into the glass which would enhance reduction. Electrode driven furnaces are a much better platform to study this and I wonder how they perform in making colored glass.

Could this be used to our advantage by minimizing the need for reducing chemicals?
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Old 04-10-2018, 06:33 PM
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I make silver opals much easier in gas furnaces. You're overthinking the entire show. One of the reasons that industry avoids silver is the furtive nature of the coloring. One day it works, the next day it don't.

I just did an 80 lb melt and in that melt the silver was changed from 32 to 30 grams. It had quite a different presentation. That's two grams in 80 lbs. Don't plan a business around numbers like this. Celebrate what works when it works.
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Old 04-10-2018, 06:47 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Thanks Pete. I'll try and keep perspective...I believe you likened it to herding cats.

I was celebrating like an idiot when it was working yesterday. I'm a glutton for punishment.
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Old 04-17-2018, 03:58 PM
Dave Bross Dave Bross is offline
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The dark amber is an overstrike. Too many hot/cold cycles or working it too long.

Have a look at your process and see if you can be finished when you've got the good color.


I would love to have some sort of magic bullet that would stretch out the strike timing/event.
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Old 04-17-2018, 04:41 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Hi Dave. I've run the chalcedony a number of times and have seen the overstrike. It usually takes quite a few cycles and (for me) rather than amber, goes to more of a nasty light brown opal.

I think what I am seeing is an overreduction because it starts clear, turns transparent amber from the pipe end and any color swirls gets swallowed up by the amber. Another tell-tale is that I've been able to tone it down by adding less and less black tin. Over-oxidized just never shows anything but a very light amber (no swirls). Pete mentioned that too much tin can just reduce everything and make it amber...sounds like my experience.

It's weird how it was not a problem when I first started. I think I've got a handle on it now and feel like I have a much better starting material using the unoxidized clear batch.
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Old 04-17-2018, 04:54 PM
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try less silver as well. Reheats aew critical. Too many lets colloidal structures to get planted. You want a confused structure. Think about speed. I can make my chalcedonia be transparent or intensely opaque based in reheats. ( in reduction) Electric kilns mess with everything.
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Old 04-17-2018, 05:50 PM
Dan Vanantwerp Dan Vanantwerp is offline
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Lower black tin, lower silver...less $$ I like the sound of that.

The work process is interesting with this glass. I just do my best...hit the marver, use my air gun, heat a little, heat a lot, let it get stone cold or just until the red glow goes dull. I try to work something out as a consistent process but then the glass changes. I'm looking for a way to make cast iron blocks like Dave because that sounds just awesome.

I've gotten some nice colors and effects...no complaints. It's a lot of fun and always has the potential for a real WOW!
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Old 04-17-2018, 06:18 PM
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The reasons industry avoids this stuff are very real.
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Old 04-17-2018, 07:16 PM
Dan Vanantwerp Dan Vanantwerp is offline
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I'm glad to not be industry. This video is pretty amazing:

https://www.youtube.com/watch?v=WX64FElTFZc
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Old 04-17-2018, 07:35 PM
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I'm glad to no longer be in industry.
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Old 04-18-2018, 10:43 AM
Dave Bross Dave Bross is offline
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Those type of blocks pop up on Ebay fairly frequently. they were the go-to solution for the West Virginia through Ohio style of glassblowing so there are a LOT of them out there.

I've also seen some pieces of cast iron glass molds on Ebay that would work well.

You can't beat them for durability. Some of the old timers Tom and I saw in the aforementioned area were just wearing down the ones their granddaddy had.
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Last edited by Dave Bross; 04-18-2018 at 10:46 AM.
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Old 04-18-2018, 05:36 PM
Tom Fuhrman Tom Fuhrman is offline
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I had several sizes of the cast iron blocks produced a few years ago. I have sold all I had but still have a company that would cast them. Only thing keeping me from making another run is the quantity I would have to have produced at one time. I'm not looking to inventory anything these days.
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