Wednesday, 20 October 2021
Wednesday, 13 October 2021
Wednesday, 6 October 2021
Wednesday, 29 September 2021
Scheduling the Rate of Ramp
Remaining Parts of the Schedule
The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.
Wednesday, 22 September 2021
Wednesday, 15 September 2021
Wednesday, 8 September 2021
Adapting the Bullseye annealing chart for other glasses.
This post is about the method to adapt the bullseye annealing chart for thick slabs to any soda lime glass as used in kilnforming.
I frequently recommend that people should use the Bullseye chart for Annealing Thick Slabs. The question returned to me is "How do I do that?" My usual response is “Substitute the numbers for your glass into the temperatures given in the Bullseye table.” This is inadequate for composing a full firing schedule, because there is no heat up rate, only annealing times and rates. So, people react, saying "It’s only half a schedule."
That is so. The heating of glass is so dependent on layup, size, style, process, and purpose of the piece. This makes it very difficult to suggest a general firing schedule. People generally find this out after some time using already set schedules for their kiln. What works for one layup doesn’t for another.
Devising a Schedule for the Heat Up
As there is no recommendation on heat up, you have to make up your own schedule for heating up the glass. This is where I can give some general advice on some of the things you need to be aware of while composing your schedule without getting into the details of scheduling for specific projects.
The most important element to note is that the Bullseye chart is based on evenly thick pieces of glass. If you are tack fusing or have different thicknesses of glass across the piece, you will need to use a thickness greater than the total height of your piece. The calculation for this depends on the final profile. It can be between 1.5 and 2.5 times the thickest part of the piece.
The end cooling rate for the appropriate thickness is a guide for the first ramp rate of your schedule. The final rate for an evenly thick piece 19mm/0.75” is 150C/270F. This could be used as the rate for the first ramp. However, Bob Leatherbarrow has noted that most breaks occur below 300C/ 572F, so more caution is advisable at that starting ramp rate. My testing shows that using the final rate of cooling that is one thickness greater is suitable for the first ramp rate. In this example that would be 120C/216F per hour.
Even though for some thinner pieces the rates given are much faster, it is not advisable to raise the temperature faster than 330C/600F per hour to care for both the glass and the kiln furniture. This restricted ramp rate protects against glass breaking in the early stages of firing. It also reduces the amount of overshoot in temperature at the top temperature of the schedule.
The second ramp to the bubble squeeze can be at the final cooling rate for the calculated thickness up to a maximum of 330C/600F.
After the bubble squeeze you can advance at up to 330C/600F rate to the top temperature. AFAP to top is not advisable to be able to maintain control of the possible overshoot in temperature that would be created. The top temperature should be such as to achieve the result in 10 minutes to avoid problems that can occur with extended soaks at top temperature.
In the example given this gives a heat up full fuse schedule similar to this:
120C/216F to 260C//500F for 10 minutes
150C/270F to 566C/1052F for 0 minutes
50C/90F to 643C/1191F for 30 minutes
150C/270F to 800C/1473F for 10 minutes
Adapting the Bullseye Annealing Chart for Other Glasses
As you have written the first part of the schedule, you can begin to apply the annealing information from the Bullseye chart to develop the schedule. The first part of the anneal cooling starts with dropping the temperature as fast as possible to the annealing temperature.
The question will arise “Why should the Bullseye annealing chart be used instead of some other source?” It is researched. It is the most thoroughly researched set of tables for fusing compatible glass that is currently available. This means that the soak times and rates for the thicknesses can be relied upon.
“How can it be used for glass other than Bullseye?” The rates and times given in the chart work for any soda lime glass, even float. It is only the temperatures that need to be changed.
The method for making the chart applicable to full and tack fusing and slumping is a matter of substitution of temperatures. The intervals between the temperatures remain the same for all soda lime glasses – the range of glass currently used in fusing, including float glass. The soak time for the calculated thickness of your glass piece will be the same as in the Bullseye chart. Note again that if tack fusing, the soak time will need to be for glass 1.5 to 2.5 times (depending on the profile) the actual thickness.
First you need to determine the annealing point of the glass. This can be done by going to the web page of the glass manufacturer. It can also be done by using an earlier blog post giving some of the critical temperatures for a range of glasses. This information has been taken from the manufacturers’ web sites and gathered together as they are sometimes difficult to find.
Use the annealing temperature from your source as the target temperature in place of the Bullseye temperature. The next target temperatures are at 55C/100F and 110C/200F below the annealing temperature.
This means that the first cooling stage will be from annealing temperature to 55C or 100F below, depending on the temperature scale you use. This gives you the target temperature for the first cooling segment.
The second cooling stage is to annealing temperature minus 110C or 200F, again depending on temperature scale. This gives the target temperature for the end of the second cool.
The third cool is from there to room temperature.
The rates for these three stages of cooling remain the same regardless of the glass. The temperatures are different for various glasses, but the range remains constant.
This gives an annealing cool schedule for 19mm/0.75" Oceanside glass similar to this:
AFAP to 510C/ 951F for 3:00 hours
25C/45F to 455C/852F for 0 time
45C/ 81F to 400C/753F for 0 time
150C/270F to room temperature, off.
This demonstrates the intervals for the three stages of cooling are the same as in the Bullseye chart, it is only the temperatures that change. The annealing soak time remains the same, even though the temperature is different.
I will repeat, because it is so important, that the thickness to be used for the anneal soak and cooling rates for your schedule relates to the profile you desire. A fuse with even thickness across the whole piece can use the times and rates as given in the chart as adapted for your glass. A tack fuse or any firing of a piece with uneven thicknesses will require a schedule as for pieces between 1.5 and 2.5 times the actual thickness of the piece depending on the profile you intend to achieve.
However, I am not going to reveal all the secrets of my research on schedules for uneven thicknesses and low temperature kilnforming. You will need to buy my e-book for the application of the reasoning and research for low temperature fusing including tack fusing.
I have included some tables for various fusing glasses and float glass. The annealing temperature is given. These show the annealing soak time for various thicknesses. You will see the rates of cooling remain the same for different glasses, regardless of the annealing temperature. Only the temperatures change according to the annealing temperature. If your glass is– or calculated to be - in between the sizes given in the charts, you can interpolate the required times and rates as the progression is regular. Or, you can obtain these already calculated in the book noted above.
Why use higher temperatures for copper foil using 60/40 than lead came using 50/50 or 40/60?
Part of this is the physical characteristics of the solder
Melting temperatures of some solders
At 40% tin and 60% lead (40/60) the melting temperature is 238
There are two separate elements at work here – the mass of solder being melted and the effects of the pasty range of solder compositions.
In soldering lead came you are melting small masses of solder with short pauses between each melting that allow the iron to partially recover. This means running the iron at 370
In copper foil you are melting much greater amounts of solder, which takes heat out of the iron more quickly than in leaded glass. The fact is that running a bead requires melting a much greater volume of solder. The iron needs to run hot to be able to consistently melt the solder without significant periods when the iron is too cool to melt the solder quickly. This is the reason that irons are run hotter in copper foil.
It still does not explain why it is recommended to run the iron hotter for 60/40 than for 50/50 as their melting temperatures are so close.
The explanation lies in the pasty range illustrated in the graph shown above. You can run an iron hotter than needed to melt the solder, because the 60/40 requires fewer degrees to cool and solidify than 50/50. This allows you to work quickly and still have a good rounded bead.
The greater pasty range of 50/50 means that you must be careful about the amount of heat you put into the solder, because the solder will continue to move for a longer time than the 60/40. The 27 difference between melting and solidification shows solidification is not instantaneous. This pasty range allows flow while the solder cools. This means that the bead will be less rounded, and it will show minor temperature differences in the wrinkled surface. If you put even more heat than the 410
Wednesday, 1 September 2021
|Example of the problem|
|closer view of one example|
- You must advance the temperature slowly. A rate of 100C per hour will be fast enough.
- You can add a bubble squeeze soak of 30 minutes at about 630C as additional assurance of removing most of the air. The bubble squeeze is done at a lower temperature than usual, as the glass is less viscous because the slow rate of advance has put more heat work into the glass.
- The top temperature should not go beyond 720C. Beyond that temperature the viscosity of the glass drops quickly and so becomes subject to bubble formation.
- Mould texture complexity
- Type of glass (opalescent or transparent),
- Heat forming characteristics of the glass,
- Viscosity of the glass or colour,