Kiln Digital Controllers

[Teague]

I have been wondering this for a while and just haven't gotten around to asking. I'm in the market for a kiln, and everyone says you absolutely have to have a digital controller. fine. But then a lot of people seem to say that they ramp their kiln up to temperature, leave it on while they work, and then "shut it off" and get their beads in the morning. Is there something I am missing? Why do you need a digital controller if you're not actually running a program, just getting it up to temp and shutting it off?

Teague

[pam]

Hi Teague, If your kiln is large enough and your beads small enough you can probably get away with doing that, but annealing is really the controlled cooling of glass at a specific rate. That's why you need a controller of some kind. Digital is nice and makes your life a lot easier, but you can do it with a controller that allows you to turn down the heat slowly. When you turn off your kiln the fastest heat loss is when there is the biggest differential between the temp inside your kiln and the temp outside your kiln, so your kiln looses the most heat at the beginning of what should be your annealing cycle. This is exactly the opposite of what you want to happen with annealing. You want your beads to cool the slowest for the first 100 to 200 degrees, and then after that they can cool faster. For my beads I allow the kiln to cool at 1 degree every minute. By doing that I feel the surface of each bead will cool at about the same rate as the inside of the bead. This is what you want to happen. If there is a big difference in the temp inside the bead and the surface of the bead, your bead will crack. I hope that helps.
Pam

[Teague]

Thank you, you explained it a little more then I had heard in the past. I'm still a little baffled about the whole situation regarding annealing though.

Teague

[pam]

So, what has you baffled? Maybe we can help.
Pam

[MikeAurelius]

Ok - I think I understand what your question is --

What we are doing with pre-setting the kiln temperature, say at 900 F all day long, is called "garaging". It holds the beads at a temperature below annealing, but above the strain point of the glass.

This ensures that the beads don't crack from heat shock (as they *might* if you were to use a bead blanket).

Then, at the end of the session, the actual program is started on the controller, which then takes the beads from 900 F, up to the annealing point, holds them there for a speciified period of time, then slowly, under "computer" control, cools them down back to room temperature and a rate of time and temperature determined by the type of glass being annealed.

I think when they say they are "shutting it off", they are referring to the fact that a lot of people do in fact turn the power off once the kiln has reached about 600 F on the downward side.

Some folks, and I mean no disprespect to anyone, don't feel comfortable leaving a hot kiln running without direct supervision. And that's just fine. However, the kiln controller is able to automatically control the kiln temperature by itself down to room temperature, if you feel comfortable leaving the kiln alone and going to bed. I do it every day - tell the kiln to run the annealing program, turn off the lights, lock the door to the studio, and drive 30 miles home.

[BillBrach]

There are two phases to proper annealing

1. The Anneal Soak - for Effetre/Moretti, this is typically done at 968 degrees. The 'rule of thumb' is 30 minutes plus 15 minutes for each 1/4" of thickness. For the typical round bead up to about 5/8" in diameter, this is 45 minutes. This is done to allow the entire bead to get to the same temperature throughout.

2. The Anneal Ramp - this is pretty much the same for all glass, except that thicker pieces must be brought down MUCH slower than thin pieces. This really isn't a concern for most beads, and NO FASTER THAN 2 degrees per minute is the usually accepted number. This ramp is between the Anneal Soak temp, and the Strain Point for the particular glass. The Strain Point is defined as the temperature that which below, no more strain will be introduced by rapid cooling. It is the point that the glass molecules are practically moitionless. For Effetre/Moretti, the Strain Point is 840 degrees. So, to properly anneal your average sized beads, you must control the temp drop from 968 to 840 at no more than 2 degrees per minute.

The Anneal Ramp is why a digital controller is virtuallly required. Most hobby kilns on the market today cool MUCH faster than 2 degrees per minute. And, if you make really large beads, or thick slumped/fused pieces, this rate of cooling must be even SLOWER than 2 degrees per minute !!

[Gayle Raymond-Smith]

Hello Everyone;
This is the first time I have ever posted a message, hope you get it. A couple of questions.
My husband and I wish to make beads with boro, we are in the process of purchasing equipment but would like your advise. Because this is our first set up, what torch would you recommend? Also we wish to buy a concentrator rather than use an oxygen tank, what brand would you recommend which would be compatible with the torch? Also the brand of kilns that would work for boro and solf glass?

Looking forward to hearing from you.

Gayle

[MikeAurelius]

Gayle, may I respectfully suggest you start a new thread for your questions, and maybe a new thread for each question?

These are very good questions, but if they are answered in this thread they will probably get lost.

[Teague]

Ok Mike, I think you caught my confusion. I thought that the temperature you kept the beads at while you were working is the same temperature that you ramp down from. I didn't realize that you had to go up first, then down, now it makes more sense. thanks.

Teague
(must save more pennies)

[BillBrach]

Teague,

The reason that some people recommend that you 'garage' your beads at a lower temp than the annealing temp, is because deformation of the beads can happen if the temp is not really precise.

Some kilns have 'hot spots' in them, and some beads in these areas might become mis-shaped. If the kiln is REALLY at 968, then this is an OK temp to garage them at 968 too.

A lot of kilns with analog controllers and pyrometers are notoriously inaccurate in their readings. I have an Evenheat Hotbox, that when new, was almost 75 degrees off at 1000 on the pyrometer. When it was displaying 1000 on the pyrometer, it measured 1075 on a seperate digital thermometer. This 'extra' 75 degrees WILL cause problems with deformation.

Lowering the 'garage' temp simply gives you more leeway below the deformation temperature.

Bill

[MikeAurelius]

Yep. As long as the temperature is ABOVE the strain point (of 850 for moretti/effetre), you shouldn't have any problems at all.

This becomes especially important if you ever want to work borosilicate glass. Some boro, especially greens and aventurine glass shift COE if overworked or overkilned. Keeping the temperature in the kiln for garaging below 1000 F (say between 950 and 975) will help protect and keep the greens and aventurines from shifting COE due to over kilning.