Wednesday 18 August 2021
Observations on Some Suggestions about Annealing
Wednesday 17 March 2021
Sintering
General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.
“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
https://en.wikipedia.org/wiki/Sintering
- A moderate rate (150°C) all
the way to the sintering temperature needs a two-hour soak at the top
temperature.
- A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
- A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.
- With the same rates and soak times, lower temperatures produce weaker glass.
- The lower the temperature, the longer the sinter soak needs to be for similar strengths. Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
- Lower temperatures produce more opaque glass. In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.
- Testing showed that annealing
as for 12mm is adequate.
- There was no advantage of annealing as for 25mm as that did not increase the strength.
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C. At the lower temperatures, the glass becomes damp on the outside and weeps water. At 670° and 690°C the outside became cool to touch but did not leak water. This observation depends on evenly and firmly packed frits.
Grain structure at 650C |
Grain structure at 690C |
Wednesday 3 March 2021
Firing multiple layers
- 25C/hr to 125 for 20’
- 30C/hr to 250 for 20’
- 40C/hr to 375 for 20’
- 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
- 150/hr to target temperature
- 200C/hr to 250, no soak
- 340C/hr to 500, no soak
- 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
- 500C/hr to top temperature.
- an initial rate of 135C,
- a second ramp of 230C,
- a third of 270C and
- the fourth of 335C instead of the rates for even layers.
Tuesday 5 January 2021
Expansion at Edges of Tack Fused Stacks
The most expansion for any thickness and at any tack profile is when the stack is placed at the edge. The further away from the edge, the less the expansion. There is no noticeable expansion of size when the tack stacks are placed 20mm from the edge. In most cases there is only a little expansion at 10mm from the edge. Although not tested, it seems that 15mm is a safe distance from the edge to avoid changing the edge.
The amount of glass in the stack being tacked to the base has an effect on the amount of expansion. This is to be expected based on the concepts behind volume control. Two tack layers can vary from two to three times that for a single tack layer depending on the profile of the tack.
The tack profile has an effect on the amount of expansion. At contour there is a greater expansion than at rounded or sharp tack fuse. This is to be expected, as there is less heat work at sharper tack profiles than at contour.
The thickness of the base has an influence on the amount of expansion too. Thicker stacks promote greater deformation of the edge at all tack levels. Thicker stacks need to be placed further from the edge to avoid changing the perimeter. Thicker stacks create greater change in the edge on single layers than double layers.
Setup for 2 layer base and 1 and 2 layer stacks at various distances from the edge. |
Contour fuse test, 6mm base
1 layer placed at edge, at 10mm from edge, at 20mm from edge, and at 30mm from edge. 2 layer stacks placed in the same way.
Fired results, outlined for clarity |
1 layer placed 10mm from edge – expansion of 0mm
1 layer placed 20mm from edge – expansion of 0mm
1 layer placed 30mm from edge – expansion of 0mm
2 layers placed 10mm from edge – expansion of 2mm
2 layers placed 20mm from edge – expansion of 0mm
2 layers placed 30mm from edge – expansion of 0mm
1 layer placed at edge, at 10mm from edge, and at 20mm from edge.
2 layer stacks placed in the same way.
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm
2 layers placed 10mm from edge – expansion of 1mm
2 layers placed 20mm from edge – expansion of 0mm
Fired result of 6mm base with 1 and 2 tack layers, rounded tack. |
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge. 2 layer stacks placed as above.
1 layer 10mm from edge – expansion of 1mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
2 layers 10mm from edge – expansion of 1mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
Fired result of 3mm base with 1 and 2 tack layers. |
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge. 2 layer stacks placed as above.
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
2 layers 10mm from edge – expansion of 0mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
Wednesday 17 June 2020
Firing Uneven Layers
Wednesday 3 June 2020
Large Bubbles
Image from B Stiverson |
Wednesday 12 February 2020
Tears in Slump
It was then slumped. When the kiln was opened after the completion of the slump, this split was revealed in the centre of the bowl, rather than a complete break.
This indicates that observation is needed when trying new layups and moulds to find the appropriate temperature. It reveals at what stage any problem occurs.
It might have been just too quick for the thickness of the piece. The piece is reported to have varied from 6mm to 9mm. From the picture this might have meant a single layer base, or more likely, a two-layer base and then the two-layer top pieces, making it 6mm to 12mm. The firing schedule would need be as for an 18mm piece in the first instance, and 24mm in the second.
If you look at the fusing schedule, the anneal soak and anneal cools were adequate for a 12mm piece, so if compatible glass was used, there should be no annealing stress in the piece. It is important to consider this, as a stressed piece can often break during an otherwise adequate slump schedule.
It is possible that the split occurred on the way up and then re-attached during the slow rise to the high working (lamination) temperatures. This would give the appearance of an incomplete split, commonly called a tear. We don’t have any information about the state of the edges. At 720°C an early split could have re-attached. The reason for considering this possibility, is the change of curve on the left end of the split. However, this does not seem likely as the split clearly shows on the edge of one of the squares, but does not go all the way across. Lamination temperatures are not high enough to seamlessly heal a crack.
It is most likely the split occurred during the rise in temperature. The reason for speculating this, is because of the distortions of the squares. If the split occurred before any significant slumping the distoritions in the squares would be explained.
The fuse schedule to the bubble squeeze temperature is sensible, but the slow rate of advance is continued to the top temperature, which is not usual. This means there was no consideration of a bubble squeeze soak, although this did not prove to be a problem. The annealing is suitable for up to 12mm.
If you have a community of kilnformers or a store you can take the piece to, you are likely to be able to give responses to questions and to get the information required about the possible problems and solutions.