Friday, 10 August 2012

Bottle Slumping


The amount of slump you want will determine the top temperature combined with the rate of advance.

You have the choice of firing slow and low or fast and high. If you choose the former, you can also choose a lower final temperature. As a starting point for considering your firing schedule you can assume that bottle glass is very similar to float glass. This means that you can start with an annealing temperature of about 548C and a softening point of 720C. The strain point is around 510C, so all the annealing needs to be finished by that temperature.

Bottle glass is fairly robust, so an initial rise of 150C/hr to 600C may be slow enough, considering the differences in thickness that most bottles exhibit and then faster to your top temperature. A little experimentation is required. The minimum temperature required for a slump will be around 720C. The slumping temperature you choose depends on how fast you want to achieve your slump and how flat you want the result to be. Slow slumps can be done at around 720C, but if you want faster or flatter you need to consider temperatures around 770 -790C. You will need to observe to determine what temperatures are best for your desired results.

Annealing needs to be done carefully for two reasons. You do not know how consistently the glass has been made and you have a range of thicknesses involved in the now slumped piece. So it is safest to assume the piece is now about 20 mm thick. This would require a 3 hours soak at 550C; an initial annealing cool rate of 25 C/hr to 495C; a secondary cool rate of 45C/hr to 440; and a final cooling rate of 150C/hr to around 50C. If the bottles are clear or light enough, you should do a stress test on them by placing the bottle between polarised filters and over a light source. This will tell you if you have annealed properly.

You also need to consider whether you can fire bottles from different brands of drink and different parts of the world together. Some say don't try it. Others say there is so little variation in glass the world over that careful annealing will compensate for the differences. Experimentation is the only way that you will be able to tell.

Allow a day after firing before any cleaning or washing of the bottles is done.