View Full Version : Batching a 1000lb Day Tank

Max Grossman
04-04-2012, 10:00 AM
I wanted to ask for some advice here. I do all the charging for our shop, a private studio with a big group of professional renters. We recently switched from Spruce cullet to batch (obviously), and I'm having issues melting such a large volume of batch. I feel like I'm getting close, but I charged Saturday night and still have tiny seeds in the glass today that aren't going away.

Our tank is a 1000lb gas fired day tank, and I typically charge 5-700 lbs once a week and do a cullet charge a couple times a week.

Our work temp is 2140, my most recent schedule is charging at 2350 and fining at 2400 for 10 hours, with a squeeze at 2000 for 3 hours. I wanted to do the squeeze at 1900 (to get the antimony to shift valence) but my boss thinks that's too low and will cause cords. Pretty sure our cords come from refractory.

I'm started to get frustrated charging the batch, I'm not sure if it's just too much mass to fine out, or if there's other factors at play, but we are a busy studio and a lot of glass gets used, and I'm finding it hard to get the furnace charged and good in time for the next day's rentals. We close at 6 and open at 10, so that doesn't leave me very much time for a long program. Sunday we are closed, but that assumes I can get the melt fined out in 24 hours. I charge the cullet at 2300 for 4 hours and that's ok for the middle of the week, but I can't get away with shorter and colder for the batch.

I'm charging 100 pounds every hour and a half, I'm not sure if that's too much but the glass is totally flat and I stir it before the next charge to make sure it's fully melted. After the fine I get the big bubbles, but the tiny seeds remain when I squeeze it down.

My thoughts are it's one of a few issues - I'm charging too much overall or too much per charge, our furnace isn't tuned properly, the squeeze isn't low / long enough, the fine isn't long enough. 2400 is really hot and I definitely don't want to go hotter. In the times I've only charged 300 pounds the glass has been great the next day, but from what my boss tells me we can't afford to charge small amounts every night, it runs up the gas bill.

So, is there any way to get a 1000 lb day tank charged and ready to go for the next day's rentals? Anyone have any ideas on how to schedule the charges or how to melt the batch properly in as little time as possible? Thanks.

Rollin Karg
04-04-2012, 10:58 AM
Your schedule looks good to me. I see two things that would concern me. Do you potato at the end ? If not, this will help seed problem.

The second thing is, 500 to 700 pounds a week in a 1000 pound furnace means it never goes to the bottom. This is going to cause cords.

Jeff Thompson
04-04-2012, 11:22 AM
I thought the whole point of a day tank was to be able to turn it up over 2500 during batching/cooking? You say you don't want to turn up over 2400 but give no reason. The freestanding crucibles can't go over 2400 because of the high alumina content.

Tom Fuhrman
04-04-2012, 02:30 PM
Actual surface area and depth of glass in the furnace can make a lot of difference as to how long it takes to fine out. If your glass is very deep it may be part of your problem. I agree with Rollin though, try the potato to stir, it won't hurt. If it's designed right, it should be no problem as I have some friends that have much larger day tanks than you and charge at 5 to 9 or 10PM and have good glass the next day. They do mix their own batch though. A squeeze at 1900 to 1950 might help though.

Pete VanderLaan
04-04-2012, 04:08 PM
what's the liner made of and how old is that liner? I think you may want to increase the time at 2400F. The squeeze is kind of cold for such a big tank to recover from fast and it may be that the valence is never switching in the lower part of the pot. Stirring will help that. When you have finished the chemical reaction, you should be seeing big blisters with clear glass in between. If you are seeing tiny seeds, the reaction is not complete. Use your eyes, don't use a clock.

On to Hungary.

Max Grossman
04-04-2012, 04:11 PM
I thought the whole point of a day tank was to be able to turn it up over 2500 during batching/cooking? You say you don't want to turn up over 2400 but give no reason. The freestanding crucibles can't go over 2400 because of the high alumina content.

I suppose I could, I just noticed that when we switched to batch and started charging hotter a couple areas of bricks above the liner seemed to take a hit. I never hit the walls with batch, so I chalked it to up to this furnace not being run at those temperatures before (it's newly rebuilt).

The tank is 18 inches deep and really wide, it's a pretty shallow tank. I don't understand why it's having trouble fining out.

Spruce says not to go above 2500, but should I try raising it from 2400 to 25? Or hotter? What do people with these kinds of tanks usually cook batch at?

Edit: The liner is new as of August of last year, and is made of AZS, I believe.

Jordan Kube
04-04-2012, 04:51 PM
It's funny what people think cause cords sometimes. We used to stir before each charge. Mixing isn't going to hurt.

Pete VanderLaan
04-05-2012, 03:04 AM
Cords can come from heat stratification among other things. 18 inch is not shallow by any means and I would think that the time spent at 2400F should be increased to allow for penetrating the mass. Keep stirring. 2500F is really awfully hot , particularly if you are seeing degradation of the wall above glass line. Try a longer 2400F first.

Hugh Jenkins
04-05-2012, 04:30 PM
Mixing some of your cullet into your batch charges can shorten charging and melt time and reduce the cords that might come from all batch and all cullet charging.

Tom Fuhrman
04-06-2012, 08:54 AM
In my opinion, 18" deep is quite deep, not shallow. unless you're tank is about 5' wide. There is a mathematical calculation used by the large furnace manufacturers as to surface area vs. depth of glass. It makes a big difference. I have some friends that have a 12,000lb. tank and they can charge one day and use and empty it the next with very nice quality glass.

Rosanna Gusler
04-06-2012, 11:51 AM
wow! what do they make that uses that much glass in one day? IMNTK. rosanna

Tom Fuhrman
04-06-2012, 01:00 PM
production shop that presses some items that can weigh as much as 15lb. ea. i.e. laboratory ware, Vacuum jars, mortars and pestals, etc. they also do a lot of clear crystal paperweight blanks for the awards industry that people put a photo under or engrave on them. One of the few big hand shops left in the US.

Dave Bross
04-09-2012, 10:43 AM

If I'm remembering correctly, the glass in those tanks was only a few inches deep at the working end? when you took me around to see all those places the shallowness of the tanks was one of the big Aha! moments.

In two of my furnaces one takes a shallow and wide pot, the other a very tall and narrow pot. The tall one takes way longer than the short one to fine.

Tom Fuhrman
04-09-2012, 11:54 AM

If I'm remembering correctly, the glass in those tanks was only a few inches deep at the working end? when you took me around to see all those places the shallowness of the tanks was one of the big Aha! moments.

In two of my furnaces one takes a shallow and wide pot, the other a very tall and narrow pot. The tall one takes way longer than the short one to fine.

That why I questioned the depth of the glass and surface area. The larger the surface area and the lower the depth the quicker the oxygen escapes as I see it.

Jon Myers
04-09-2012, 12:02 PM
So you don't think those tiny bubbles that wont go away are reboil? I've only noticed them when melting too hot...

Rahman Anderson
04-09-2012, 05:03 PM
Charged many a day tank. Never even took Sundays off. Turn up around three done between 10-1. Seeds in the morning but fine by noon the next day. I think 2500 sounds too hot. Take a gather before you charge again. It should be flat with small bubbles in the tank. Gather and pull a stringer. How big are the chunks of batch that have not melted? Big as a pencil lead? Big as pea? Not sure how to direct you but if you do it before each charge you can see where you are at. Maybe more time between charges. One of the tanks I charged for years had no controls. Just a gate valve and an old broken optical pyrometer. After the last charge just let it get to about 2400 then set back the valve to the run position. Turn it off and open the door for 20 minutes in the morning for a "squeeze". That furnace we piled as high as possible every charge and waited till the glass was ready. 2 and a half hours for the first but only an hour for the last charge. Good luck!

Jordan Kube
04-09-2012, 11:24 PM
And made some of the most expensive stuff out there!

Greg Vriethoff
04-10-2012, 03:18 AM
...is there any way to get a 1000 lb day tank charged and ready to go for the next day's rentals?

I did this for four years at one of the larger volume shops in Seattle. Same thing. 1000 lb day tank, SP, and owner and renters that demanded bubble-free glass the next morning. On several occasions I charged up to 750 lbs. in one night

So to answer your larger question, yes, there is a way to do this.

I was only the charging monkey/floor sweeper, so I didn't have anything to do with setting firing profiles or making adjustments to the furnace (on rare occasions, someone more knowledgeable would walk me through things on the phone, but I couldn't tell you now so many years later what any of it was).

Most of what I can tell you about doing the actual drops has been covered by others already, and from what you've already offered, it sounds like your charging methods are sound.

We had a seed problem for awhile at one point during my tenure. Stirring helped a lot. It's especially important after the first drop (if you're charging on top of "old" glass) to prevent stratification (which helps more with preventing cords, I believe). I never used a potato.

I'll give some details of my methods (or, more precisely, the methods I was instructed to employ) in case anything shakes loose.

When I did my drops I'd most often do 150 lbs every hour and a half to two hours.

When charging I'd either use a scoop/shovel-like thing that held 10-15lbs, or an open chute that held 50lbs. I never used bags. I'd always add some cullet to each drop (if available).

I would avoid piling it up in the center of the tank, and do my best to spread/distribute the entire mass evenly over the surface of the glass without touching the exposed liner with raw batch. It's important to not drop a large mass of batch in the center of your tank (or crucible) as you can end-up with a ball of unmelted batch lurking just under the surface. The part you can see may look nice and smooth, but, like an iceberg, most of the mass is below the surface.

After the surface was smooth I would stir and let it soak for about another 10-15 mins (or however long it took me to set up the next drop). The stir would also give me an indication of how the melt was going by what the glass looked like on the rod. If it still looked milky or frothy, I knew I was going to need a few more minutes of soaking and then another gather to see if it looked right before adding more.

The only other piece of advice I'd like to give that I think is very important, is to keep good records of what you're doing. In addition to noting time, temp., and lbs., you should write down the production numbers that are stamped on the bags. From what I've heard, SP cullet doesn't have these, but apparently the batch bags still do. All of this information is important in helping determine if the problem lies with the operator, the equipment, or the materials themselves.

Keeping a charging log will pay-off in the long run.

Pete VanderLaan
04-11-2012, 11:56 AM
Keeping a charging log will pay-off in the long run.
Amen. Don't use a clock....

Hugh Jenkins
04-11-2012, 02:40 PM
Another thing to put into a charging log:

If you need to change something, put it into a note what you are planning to change, why, and what you are looking for as a result. Change one thing at a time if you possibly can. And make a note of the the observations and results from that change for better or worse.

When you have those conversations that start "remember when we tried ..(smaller charges, the batch we got from Joe, etc. etc. etc,) there is a way to find that if there are notes. Just charges and temps and time doesn't tell the whole story of what was going on.

And the note that says "this was a perfect melt!" is a valuable record to have too!

One thing that charging too hot can do is to evolve a lot of the CO2 gas and even some of the O2 from the antimony and nitrate during the initial melting. You need to have those gases still in the glass when you finish charging in order to create the larger bubbles that sweep the melt.

Dudley G did some of the first real observations of melting on the cooler end, around 2150 and then heating the whole thing up at the end of charging. I believe after years of following that basic plan, that way less cords, way less tank or crucible attack, and way more consistent melts are the result. I also think that mixing the cullet into the charges is better than piling it in all at once.

Pete VanderLaan
04-12-2012, 02:42 AM
More amens:

If you see cords in a fresh melt that you know was mixed well but melted hot, you are most likely actually seeing the liner in the glass. The Corning Protocols always said to melt as cold as you could get away with, not as hot.

Max Grossman
04-13-2012, 11:55 PM
Well, to give you guys an update, this week's charging has gone much better. Still charging the same amount, about 700 pounds Friday night, but I'm preheating the furnace for a lot longer, charging a little hotter, and melting at 2400 for 10 hours with a 4 hour squeeze at 1950. I'm also starting out with charging 75 pounds until the tank heats up sufficiently, and then moving on to 100 pound drops.

Between that and waiting a bit longer inbetween charges, our seed problem has almost completely disappeared. There were tiny seeds on Monday morning in some of the glass, so tonight I'm stirring more and trying to get the melt almost free of unmelted batch before each charge. I suspect our gas line might not be high enough pressure and our furnace might not be tuned / insulated perfectly, which is why it's taking so long, but it's worth it to spend the extra time making sure it's totally melted and homogeneous. I did a mid-week charge of 300 pounds of batch with the same program, and the glass was perfect the next morning.

I think between decreasing the size of each charge, stirring more, and squeezing a bit lower and longer, I'm on my way to getting a perfect melt every week. Thank you guys for great advice!

Pete VanderLaan
04-23-2012, 08:34 PM
That ( deleted Post) is a really amazing collection of bad ideas. The presumption here would be that reboil is actually occurring. It's not. When the glass is melted, the valence of the antimony is monovalent. In the squeeze, it flips to pentavalent. Reboil only occurs when the glass flips back, having been taken above about 2175F.

The presumption here is that Dino ( who I know) is using the same formula that Max is using. He's not. If you did the things you propose with SP87 you would have 1000lbs of stuff to take to the dump.

Hugh Jenkins
04-23-2012, 11:20 PM
Pete, what works, works. We also presume that all of our thermocouples read the same, that our pressure gauges are all accurate, etc. I don't think that lots of preheating is necessary, but this is a deep furnace and thermal mass is an important consideration in deep pots and tanks. Hard to get the heat down there when you have a layer of insulating batch foam on top of it. Smaller starting charges, more time between charges, longer heat time, more complete cooling, all sound like good steps to me. 2400 though is screaming hot on the tank.

This was a good thread on solving a melting problem with none of us there to observe the process. Any five of us would probably come up with a somewhat different methods as a solution. I agree, it's not reboil that makes the seeds.

Good on ya, Max.

Rick Wilton
04-23-2012, 11:34 PM
Pete was commenting on a post that made some different suggestions. That post was then deleted. So Pete's post doesn't have the context required.

Pete VanderLaan
04-24-2012, 03:23 AM
Different glasses melt totally differently. Much Italian glass that is melted, which was the glass mentioned in the deleted post, is really high in expansion. It is really a rare glass that can expand at 104 and not be really chock full of alkaline fluxes at the same time and subsequently melt really easily and colder. I have some glasses chock full of fluorine that are 96 L.E.C. but are ready to use in a mere three hours after the final charge. By the same token, I have 96 glasses that take forever to melt because of the high zinc and cadmium content. Generalizing about melting is dangerous.

The question here was about Spruce Pine batch. I don't care who is melting SP87, putting batch on top of partially melted batch is a recipe for disaster. The stuff about reboil, you have already covered as did I. A little knowledge can yield dangerous advice. Allison, If I was too harsh, I regret that and apologize.

Rollin Karg
04-24-2012, 11:51 AM
2140 seems like a very high working temperature. Do you think that's accurate ?

Jordan Kube
04-24-2012, 04:46 PM
Depending on the furnace, Rollin. TC placement, calibration etc. I've routinely worked out of furnaces at 2150.