Wednesday 12 May 2021
Materials for making dams
Wednesday 5 May 2021
Colour Dilution of Powders
Credit: www.warm-glass.co.uk |
Wednesday 28 April 2021
Sharp points on rectangles
At
the conclusion of firing pieces with right angles or sharper shapes you often
find very sharp needle points at the corners.
Wednesday 21 April 2021
Soaks Below the Softening Point
Saturday 17 April 2021
Effects of Dams on Scheduling
I recently made a statement about the effects of various dam materials on the scheduling. This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board. I decided to test these statements. I found I was wrong.
I set up a test of the heat gain and loss of the three materials. This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams. The dam materials were laid on the kiln shelf with thermocouples between. These were connected to a data logger to record the temperatures.
The schedule used was a slightly modified one for 6mm:
300°C/hr to 800°C for 10 minutes
Full to 482°C for 60 minutes
83°C to 427, no soak
150°C to 370°C, no soak
400°C to 100°C, end
The data retrieved from the data recording is shown by the
following graphs.
Highlights:
·
The dam materials all perform similarly.
·
This graph shows the dams have significant differences
from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in
this case).
·
There is the curious fall in the dams’ temperatures
during the anneal soak. This was
replicated in additional tests. I do not
currently know the reasons for this.
·
The dams remain cooler than the air temperature
until midway during the second cool when (in this kiln) the natural cooling
rate takes over.
·
From the second cool to the finish, the dams
remain hotter than the air temperature.
Some more information is given by looking at the temperature
differentials (ΔT) between the materials and the air. This graph is to assist in investigating how
significantly different the materials are.
This graph is initially confusing as positive numbers
indicate the temperature is cooler than the material being compared and hotter
with negative numbers.
As an assistance to relating the ΔT to the air temperature some relevant data points are given. The data points relate to the numbers running along the bottom of the graph.
Data Point Event
1 Start of anneal soak.
30 Start
of 1st cool (482°C)
45 Start
of 2nd cool (427°C)
65 Start
of final cool (370°C)
89 1st
55°C of final cool (315°C)
306 100°C
At the data points:
·
At the start of anneal soak the ΔT between the
dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.
·
At the end of the anneal soak of an hour, the
air temperature is 20°C higher, although the ΔT between the dams has reduced to
12°C.
·
At the end of the 1st cool the ΔT between
the dams has reduced to 9°C and the ΔT with the air is 3°C.
·
At approximately 450°C the air temperature
becomes less than the dams.
·
At 370°C the hottest dams are approximately 17°C
hotter than the air. The ΔT between the
dams is 10°C.
More generally:
·
The air temperature tends to be between 17°C hotter
and 17°C cooler than the ceramic dams during the anneal soak and cool. The difference gradually decreases to around
8°C at about 120°C.
·
Ceramic and fibre dams loose heat after
annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a
peak difference of 9°C at the start of the second cool. This means the heat
retention characteristics of ceramic strips and fibre board are very close.
·
Between the annealing soak and about 300°C the
vermiculite is between 12°C and 9°C hotter than the same thickness of fibre. Vermiculite both gains and loses heat more slowly
than the ceramic or fibre dams do. This
means that vermiculite is the most heat retentive of the three materials.
Conclusions
·
Dams will have little effect during the heat up
of open face dammed glass. The slight
difference will be at the interface of the glass and the dams where there will
be a slight cooling effect on the glass.
Therefore, a slightly longer top soak or a slightly higher top
temperature may be useful.
·
The continued fall in the dams’ temperature
during the anneal soak indicates that this soak should be extended to ensure
heat is not being drained from the glass by the dams to give unequal
temperatures across the glass with the risk of inadequate annealing. I suggest
the soak should be extended to that for glass of 6mm thicker than actual to
account for this.
·
The ability of ceramic and fibre dams to absorb
and dissipate heat more quickly indicates that they are better materials for
dams than vermiculite board. The
slightly better retention of heat at the annealing soak, indicates that ceramic
is a good choice when annealing is critical.
Based on these observations, I have come to some conclusions
about the effect of dams on scheduling.
·
There is no significant effect
caused by dams during the heat up, so scheduling of the heat up can be as
for the thickness of the glass.
·
The lag in temperature rise by the dams indicates a slightly
longer soak at the top temperature (with a minor risk of devitrification), or a
higher temperature of, say 10°C can be used.
·
The (strange) continued cooling of the dams during
the annealing soak indicates that extending the soak time to that for a piece
6mm thicker than actual is advisable.
·
The cool rates can continue to be as for the
actual thickness, as the dam temperatures follow the air temperature with
little deviation below the end of the first cool.
·
Ceramic dams perform the best of the three
tested materials.
Wednesday 14 April 2021
Smooth Surfaces on Drop Vessels
Wednesday 7 April 2021
Firing Records
Bullseye Glass Company |
To develop your fused glass practice, you need to record lots of information about your firings. This tells you what has gone well and not so well. It hones your expectations about how you should be preparing, scheduling, and analysing your experiences. It becomes your detailed memory bank of results and gives directions for the future. This should be done whether fired in your own kiln or someone else’s.
Categories of information for the record
There is quite a bit of information that needs to be
included in such a record. This is my
view of what needs to be included in
your logbook for future reference.
Record the date of the firing as that will give you historical information on similar projects. It can show you what you have changed over time and the variations you have introduced.
Glass used
This is not only the type of glass (Bullseye, Float, Oceanside, Wissmach, Youghiogheny, etc), but the colours used. This should include the manufacturer’s code numbers to enable you to replicate the glass used.
This can be a description, a drawing or pictures of the set-up of the piece prior to firing. This is vital to later understanding what you did in this firing. Record any glues or stabilising elements you use. Any frits or powders used should be recorded. The placement in the kiln is important - centred, one corner or another, level/ height in kiln, etc., can affect the results. You can make a sketch or take a photo to attach to the record rather than writing separate descriptons. How it comes out is recorded later.
Dimensions
The dimensions (h x w x d) including any variations in height are needed to compare with other projects. This might be included in the lay-up diagrams or pictures, but it is most useful to have the dimensions and their variations recorded as numbers too. You might think in terms of layers, but remember to record the thickness of each layer/piece (e.g., 2mm, 3mm, 4mm, 6mm, etc)
This is especially important if the kiln is not yours. Every kiln has variations and it is important to compensate for that in scheduling and placing of the piece in the kiln.
This is essential in gaining an understanding for planning any modifications. The process can be described by standard terms - e.g., sinter, slump, tack, contour, full fuse, casting, melt – or by your own terminology (if it is consistent).
A statement of your project and aims is very useful for the future. It is a reference point to use in comparing what you wanted with the results of the firing.
This includes essential information affecting the firing – shelf type (e.g., fibre, mullite, ceramic tile), mould type (e.g., ceramic, fibre, steel), and a description or sketch including any reference codes.
Kiln furniture. The kind and quantity of kiln furniture (dams, stilts, posts, etc) can affect the firing results, so need to be recorded.
Separators
This includes kiln wash (type, whether new or the number of uses), fibre paper type and amount, mould coatings, and anything else you may use to keep the glass from sticking.
Schedule
This is the thing most everyone remembers to record. You need to record it each time you use it – even if you have used it many times before. You need to record each step of the program.
So many times, people report that “it [the schedule] has always worked before”, only to discover that some element had been intentionally or accidentally altered from past firings. I normally write the schedule in a logbook and then enter it into the programmer. I use the written record to check against what I have entered into the controller. Then I know I have programmed what I intended. I can also check on earlier, similar firings to see the variations I have used in the past.
Drawings or pictures of the finished item are essential. A description of the results is also needed as a picture does not tell the whole story.
You should also give a commentary on the results of the firing. This should include successes as well as disappointments. Thoughts for future similar firings should be written down. They will be forgotten soon, if you don’t.
As you can see there are many elements that need to be recorded as they each can affect a firing. I see these as a minimum, and you will add elements important to you for this list.
It does not matter much in what form you keep the information. It can be a ledger, spreadsheet, database or your phone or tablet that you carry with you always. There are several apps for recording the kiln firings that can be used. What is important is that you can record the information immediately, or as you prepare the work for the kiln, into the chosen form of recording. I use a logbook and convert that in my leisure moments to a spreadsheet (usually at new years day). This allows me to compare information over time and especially the kinds of firings that I rarely do. It also allows me to search by various processes.
It is important that you back up any electronically held information to the cloud or other device to protect against loss or corruption.
It is useful to have a form for compiling this record. A number of elements of the records can be reduced to tick boxes to ease the recording. It helps to remind you of the information you need to log for each firing. Bullseye have an excellent form that you can use or adapt to your needs. There are a few apps that can be used on phones or tablets which are useful for those who record everything on their phone. Remember to back it all up to the cloud for preservation in case of loss or damage.
Wednesday 31 March 2021
As Fast as Possible Firings
This is just another small element in why moderate ramp
rates can be helpful in providing consistent results for the glass.
Darkening leads
Wednesday 24 March 2021
Contacting the Wholesaler
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