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Old 01-15-2004, 05:17 PM
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WARNING! LONG POST!

Also - I'm no expert (having only done one batch to date, but I figured "this is really fresh in my mind", so ...

Here are my batch cooking instructions for Spruce Pine (87) Batch using my 40lb wire furnace:

NOTE: since this was the first batch run since the rebuild, the crucible started totally empty at about 27F. Subsequent charges will start with a non-empty crucible at 1919F (my "squeeze and hold" temperature) A non-empty crucible will have anywhere from 1" of glass to 1/2 full depending on what I need and when.

1. The crucible is heated to 2150F. This was done using the ramp function (to preserve elements) of 100F/hour.

2. Once the crucible has equilibrated at 2150F (say 2-3 hours), batch charging begins.

Batch is added such that about 1/4 of the crucible volume is added at one time.

Temperature should initially drop almost 20F after an addition, but as the crucible fills this will change to a drop of maybe 2 degrees F (as the glass mass increases and equilibrates at 2150).

Temperature recovery times will also decrease as the crucible fills, from maybe 10 minutes to under 30 seconds.

Once a charge of batch has been added, the furnace is left for between 2 to 2 1/2 hours to allow the batch to melt flat. Under no circumstances should batch be added until the previous charge is melted totally flat. It should look like slightly foamy glass - it should NOT look like raw batch at all.

The crucible should be filled NOT FULL. I found for a cylindrical crucible that 1 1/2 inches below the rim is more than enough. Any higher and the cooking process (heat) will cause the glass to expand and overflow. This is, of course, very crucible dependant. Larger crucibles with non-vertical sides have much more leeway than the small vertical pots.

3. Crucible is filled and the glass is flat - time to cook. I heated the glass from 2150 F (charging temp) to 2250F, again using my 100F/hour ramp function on the controller (Watlow).

In my furnace, it takes about 4 to 4 1/2 hours for the entire 40lb pot of glass to fully equilibrate at the new temperature. This can be checked by peeking at the glass. If the temperature drops more that 2-3 degrees F and/or takes more than about 30 seconds to recover, you are not at equilibrium.

Once at equilibrium, I let the glass cook at 2250 for an additional 12 hours. After 10 hours, you can check the glass to see if it's "done". I use two tests to determine the "done-ness" of my glass. First, a visual inspection should show a very even glass mass, consisting of very fine bubbles (pinhead sized). Second, I take a gather of this glass - the glass should be easy to gather, and the gather should have an even distribution of many pinhead sized bubbles. Other than the bubbles, the glass should appear "clear".

NOTE: I cannot stress enough the value of SSR's and a good controller with a ramp function. Although you don't "blast" the temperature up as fast, you preserve the elements because you are always raising the temperature by small increments, allowing the controller and SSR to give small "bursts" of power to the elements instead of one large-amperage blast. This helps preserve the elements.

4. WHen the glass is done, I squeeze it by quickly cooling the furnace (ramp turned off) to 1919F and leaving it overnight (at least 12 hours). Resist the urge to peek at the glass once the temperature drops below 2200F, as this will chill the surface of the glass. Just leave it.

5. Once the glass has been at 1919F for 12 hours, you can bring it up to working temperature, or let it sit until you need it. For me, working temperature is 2070F. At that temp, the glass was easy to gather, but not "watery". This is somewhat of a personal choice - you want the glass at a temperature that produces nice gathers, but still hot enough that any trails quickly melt away from the punty (as you rotate it on the pipe stand near the crucible).

6. After your blowing session is complete, you can either heat the crucible to 2150F to start charging again, or cool it to 1919F to hold it for another day (depending on the amount left and what you need).

FINAL NOTES: If you are going to shut off the furnace, then you must gather as much glass out of the crucible as possible. I've found with the cylindrical crucibles you can get all but about 1/8 inch out of the pot with a small amount of care. This will cool and reheat without problems (again, I use the ramp at 100F/hr for shutdown and startup). When starting from room temp, I also stop the heating at 900F, 1000F and 1100F for 2 hours each temperature to ease through the quartz inversion.

I've also heard of people "idling" the furnace at 1600F without problems. For now, I'm going to stick with 1919F.

Cheers,

-Richard
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