Wednesday, 21 June 2017
Wednesday, 14 June 2017
|One of Karl Harron's deep slumped bowls|
To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould.
Wednesday, 7 June 2017
The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool.
Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point. Of course, after that initial cool, the speed of cooling can be increased.
- soak long enough to get all the glass to the same temperature and secondly, to
- cool slowly enough to maintain the even temperature distribution throughout the glass.
High temperature soak
Low temperature soak
However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.
Wednesday, 31 May 2017
Another possible cause of delayed breakage is inadequate annealing. Most guidelines on annealing assume a flat uniform thickness. The popularity of tack fused elements, means these are inadequate guides on the annealing soak and annealing cool. Tack fused items generally need double the temperature equalisation soak and half the annealing cool rate. This post gives information on how the annealing needs modification on tack fused items.
Wednesday, 24 May 2017
Wednesday, 17 May 2017
These tests must be combined with an annealing test. This conists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Cut a strip of base glass ca 25mm wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm to separate the colours.
Cut 25mm squares of the colours to be tested
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Add a stack of two layers of clear to the kiln before firing. This is to test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm clear tile
Cut two strips of glass 25mm wide and 100mm long for each test
Cut two rectangles of 25 by 50mm of the same glass for the two remaining sides
Cut a square of 50mm for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters
Contingencies - All repairs have uncertainties. You do not always know what the progress of repairing will reveal. You can agree with the client that any work required in addition to the initial agreement will be notified for the client to decide whether to proceed or not. However, you can take on the risk. This is what the contingency is for. You need to build allowance for these unforeseen developments. A 10% to 20% of the total costs addition to the price is sensible if you are taking the risk.
This may all sound like it is too much trouble for a simple repair. Yes, it does take a bit of consideration to start with. But once you have established the basic labour, travel, overhead and profit levels, the rest is pretty straight forward. You will have an idea of how long it takes to do the work, to travel, the glass costs, etc., and the profit level. You only need to multiply by the rates you have established to give you the price. I should warn you - it will be much higher than you initially thought.
Wednesday, 10 May 2017
- Uroboros FX90 has an annealing point of 525C compared to Bullseye (516/482C), and to the Wissmach 90 anneal of 510C.
- Wissmach 90 has a fuse temperature of 777C compared to Bullseye's 804C.
- Another example is Kokomo with an average CoE of 93 which has an annealing range of 507-477C and slumps around 565C.
- There is a float glass of a CoE of 90 that anneals at 540C and fuses at 835C.
- Artista (which is no longer made, except in clear) had a Coe of 94 with an annealing point of 535C and fuse of 835C, almost the same as float with a Coe of 83.
Wednesday, 3 May 2017
You can use kiln washed wire, mandrels, or tooth picks which you can pull out after cooling. These tend to leave a residue of the kiln wash behind. So this is best used on opaque items.
You can use rolled or cut fibre paper, which can be washed out after cooling, leaving a clean hole. This is works well on transparent items.
Both these methods tend to leave bumps over the channel. So you can make a three layer piece. Cut the middle layer short enough to allow the element to keep the hole open (toothpick, cut piece of fibre paper, wire etc.) to be placed with enough overlap of the top layer to catch the bottom layer. In this kind of setup you need to make the top layer a bit longer than the bottom layer. Make sure you are generous in the length of the "hole keeper" so if the glass (now possibly 9mm) does expand you do not trap the material inside.
Of course on a three layer set up like this you could use thin glass which would give you about 6mm of thickness thus eliminating the spread due to volume. In this case you would need to use fibre paper or wire that is about 1.5mm high/thick. It is probably best to have a thin piece of glass on each side of the “hole keeper” to ensure the glass does not retreat due to lack of volume.
You can experiment with a layer of standard and two of thin in various combinations to find the one you like best.
Once you have fired upside down, you will need to fire polish the surface again. Do not despair at multiple firings. A lot of people fire their pieces many times to achieve the effects desired.
Wednesday, 26 April 2017
Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.
Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use.
If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.
You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container. Be careful to clean the spray container immediately, as borax crystals form quickly.