What Is a Firing Schedule

[Ian]

A firing schedule is a series of steps taken to heat glass according to the type of project you are undertaking Let's take it in steps

1.  The initial segment of the firing is there to slowly take your glass up to the the required temp to avoid breaking it by heating too rapidly.
This is known as the first segment or initial heating cycle [Called a cycle because your kiln cycles on and off all the time if you are using a digital controller] This is shown on the graph as segment 1. You will notice that it takes approx 2 and half hours to get to the end of the first segment because the controller is set to heat up the kiln at 220 degrees per hour.
2. When the temp reaches the target temp you have set the controller to for the 1st segment, it the goes into a Hold at that temp for however long you have stipulated. This is so that the glass and shelves in the kiln have time to all equalize to the temp at the top of the first segment. In this example 590 Deg. This is also called a heatsoak.

3  When the hold/heatsoak time is up the controller then continues upwards at the rate of degrees per hour you have stipulated. Now we are in segment 2 When it reaches the top temp for this segment 2, it then goes into a hold again, in this case for 30 min to allow the glass to settle and push all the bubbles out from between the two layers [See FAQ What is a Bubble Squeeze]
This segment is also known as the Bubble Squeeze segment

4. Now the kiln can race ahead to the very top temp process that you have specified in this case 780 deg. We are now in Segment 3 This is usually written as AFAP [as fast as possible] and refers to the rate at which the kiln must now carry on heating as opposed to when you set the rate of heatup in degrees per hour. When the top process temp for segment 3 has been reached we go back into a hold for 10 min as shown on the graph in the drawing This hold allows the glass to fuse together and end up with nice rounded edges.
 [Some controllers use degrees per minute as a unit of measurement of rate} When this hold time is up then it goes into segment 4

5  This is the cooling or down cycle and also takes place AFAP down to the temp set for the start of the annealing cycle {See FAQ What is  Annealing} Here another hold is applied to even the temp out again and then the Annealing cycle starts. When this cycle is finished then the controller switches of the kiln and the kiln then cools naturally to room temp and your project is done. I hope this explains the terms used by fuser's when ever they discuss firing schedules  
Ian
 

[Kev]

In addition to the above, you will also see the term "Ramp", this refers to the process of increasing or decreasing temperature, as in Ramping Up, or Ramping Down.

[Ian]

  Good catch Kev I can't believe I forgot that.
Ian

[Kev]

LOL...no prob...that's the beauty of everyone being able to add things to discussions.

[Stephen Richard]

Ian,
I disagree with so much of what you have written, that I find it difficult to know where to start.  So I will take your text as a starting point, adding those things that occur to me. What I have to say is, of course a matter of opinion and needs to be seen as a different view of the whole question of what a schedule is.

A schedule is related to what you want to achieve.  It sets the rates of increase in heat and any soaks that may be required for a variety of reasons.  It sets the top (or working temperature) and time.  It determines the amount of heat put into the glass.  It controls the fall in temperature after the heat work has been achieved, and any soaks required during the cooling stage.  The schedule is subordinate to the purpose of the firing.
Quote from: Ian on 29-05-2010, 14:04:30
A fusing schedule is a series of steps taken to heat glass according to the type of project you are undertaking Let's take it in steps

1.  The initial segment of the firing is there to slowly take your glass up to the the required temp to avoid breaking it by heating too rapidly.
This is known as the first segment or initial heating cycle [Called a cycle because your kiln cycles on and off all the time if you are using a digital controller] This is shown on the graph as segment 1. You will notice that it takes approx 2 and half hours to get to the end of the first segment because the controller is set to heat up the kiln at 220 degrees per hour.

The initial heating may take several segments, depending on what you are doing.  A thick piece or one fired many times, might be taken up in a number of stages - initially very slowly (with or without holds - also known as soaks), and then at more rapid increases.  E.g. a 6mm piece suspended on a round mould for a drape, requires quite a different schedule than one suspended on a ring for a slump (or drop).  [As you see the terminology still is not standardised.]  My experience has shown an initial rise of 50C/hr to 100C with a 10min soak, then 100C/hr to 250C, 10 mins, then 150C/hr to 500C, with 10mins and finally 200C/hr to forming temperature - in the region of 630C - 677C with an appropriate soak to achieve the effect desired - peeking is required to determine the length of this soak.  [I prefer the term "soak" as I am thinking about the amount of heat I am putting into the glass]

2. When the temp reaches the target temp you have set the controller to for the 1st segment, it the goes into a Hold at that temp for however long you have stipulated. This is so that the glass and shelves in the kiln have time to all equalize to the temp at the top of the first segment. In this example 590 Deg. This is also called a heatsoak.

Not every segment requires a soak.  When this is the case, the time is set to “0”

3  When the hold/heatsoak time is up the controller then continues upwards at the rate of degrees per hour you have stipulated. Now we are in segment 2 When it reaches the top temp for this segment 2, it then goes into a hold again, in this case for 30 min to allow the glass to settle and push all the bubbles out from between the two layers [See FAQ What is a Bubble Squeeze]
This segment is also known as the Bubble Squeeze segment

Of course, the bubble squeeze is not always done as a single point soak –although this is the way I do it.  As noted in Bubble Squeeze segment this process can be done as a slow increase in temperature over the 50C slumping range.  Also sometimes there is no need for a bubble squeeze, so the soak/dwell/hold time would be “0”.
4. Now the kiln can race ahead to the very top temp process that you have specified in this case 780 deg. We are now in Segment 3 This is usually written as AFAP [as fast as possible] and refers to the rate at which the kiln must now carry on heating as opposed to when you set the rate of heatup in degrees per hour. When the top process temp for segment 3 has been reached we go back into a hold for 10 min as shown on the graph in the drawing This hold allows the glass to fuse together and end up with nice rounded edges.
 [Some controllers use degrees per minute as a unit of measurement of rate} When this hold time is up then it goes into segment 4

Here is a real problem. Many people seem to advocate AFAP advances in temperature.  This means the kiln power is turned on to full power until the desired temperature is reached.  This will cause the kiln to overshoot the desired temperature by an unknown amount.  This is because your controller does not have time to react to the speed of increase.  Phil Hoppes gives a very good account of the effects that AFAP heat rises on kiln controllers.
http://glasstips.blogspot.com/2010/04/rapid-heat-rises-and-their-effects-on.html
AFAP is not a good idea if you want to carefully control the effects on your pieces.

5  This is the cooling or down cycle and also takes place AFAP down to the temp set for the start of the annealing cycle {See FAQ What is  Annealing} Here another hold is applied to even the temp out again and then the Annealing cycle starts. When this cycle is finished then the controller switches of the kiln and the kiln then cools naturally to room temp and your project is done.

The description of a firing schedule has to include the annealing and cooling cycles, as they are essential to the longevity of the piece you are making.  A schedule is the whole firing sequence, not just the heat up to working temperature, however that is achieved. 

The reason for cooling AFAP from the working temperature to the annealing point is to avoid as much devitrification as possible.  AFAP in this context means the controller switches off the power to the elements until the desired temperature is reached. 

The soak at annealing temperature is to allow all of the glass to be at the same temperature. This is why you will get different soak times given for different thicknesses of glass.

Once the glass is the same temperature throughout, you can begin the annealing cool. This is the temperature range where the annealing actually occurs, so care is needed to use the right rate for the thickness of your project.  What is required is a gradual, but steady decline in temperature to allow the glass to reduce in temperature evenly throughout its thickness. This even reduction in temperature should continue to the strain point and slightly below.  Typically for a 6mm piece, this could be at 80C/hr.  The strain point can often be taken as 50C below the annealing point.

Then the cooling to room temperature can begin.  This should also be done at an even rate, although faster than the annealing cool.  Too fast a cool below the strain point can cause thermal shock and therefore breakage.  Typically the cool to room temperature from the strain point can be two to three times faster than the annealing cool.  It is a good idea to control this cool to at least 100C.  If your kiln cools more slowly, it will not be using any electricity, but does protect against too quick cooling.

I hope this explains the terms used by fuser's whenever they discuss firing schedules.

   
Ian

Of course, if we want to discuss terms in use and their variations, that will make for an amusing thread.

Steve

[Stephen Richard]

Of course, we could view the graphed schedule as a mountain with a very steep left face sometimes with a small ledge before the steeper climb, and a very different right face.  As we approach the bottom of the right side of the moutain, there is a relatively steep incline, followed by a shallower one, until we reach a plateau beneath a very steep rise to the top.

The controller requirements for the right side of the mountain are in reverse to the approach - steep fall, plateau, gradual fall, steeper fall to sea level.

That is just how it looks, not why it looks that way.

Steve

[Edited after reflection on how the "mountain" looks.]

[Becki]

Ian gave the basics of a firing schedule so that the non-fusing members here would have some idea of what we're talking about when we mention schedules. Stephen expounded on the topic. Are either one of them wrong?  No. As in many aspects of glass work there is more than one way to skin a cat but the basics are pretty much the same. As Stephen said, many people advocate the AFAP approach to reaching process temp. That he chooses not to do it does not make him wrong nor is Ian wrong if he chooses to do it. Yes, there are many variables to consider when determining a schedule as well as the results you want to achieve. Not everyone achieves them the same way.

For a little more reading and because someone asked for informational links I'm adding a link to the BE Technote 4 on heat and glass.
http://www.bullseyeglass.com/pdf/technotes_tipsheets/TechNotes_04.pdf

It also includes another view of the "mountain" as well as diagrams of the affects of heat on different layers of glass.  Enjoy!

[Anne]

even us fusing members are getting something out of this.. Thanks to all of you.

[ct4mom]

Yes and us new fusers are learning quite a lot as well.

[Kev]

This is a good thread! Lots of info, opinions and ways of doing things. Though one may choose to do one over the other, at least they will know why they are choosing that route.

Good work guys!

[Ian]

Stephen

You said  "I disagree with so much of what you have written, that I find it difficult to know where to start." [How positively Dramatic]
then going on to say
"A schedule is related to what you want to achieve."
And I said
"A fusing schedule is a series of steps taken to heat glass according to the type of project you are undertaking Let's take it in steps"

 Where's the disagreement here. We are both starting off from the same point.

Where we diverge is that I was trying to explain what a schedule is to illustrate the terminology most commonly used by fusers when talking about schedules
So I came up with using a generic schedule with a visual aid and then tried to use as many terms as possible such as AFAP, upcycle, downcycle, annealing cycle, segment, etc.etc.
It was by no means intended to be a learned treatise on the way to use schedules to manipulate glass when fusing, slumping etc.

This should have been readily apparent to anyone if they took the question "what is a firing schedule" and linked it to my last comment which was,

"I hope this explains the terms used by fuser's whenever they discuss firing schedules."

Ian