Is it Possible?
It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you. There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.
Preparation of the Glass
Prepare the edges to their final finish before slumping. This because the slumping temperature will not be enough to alter the finish of the edge significantly. This preparation can be done with diamond hand pads, or wet and dry sandpapers. Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit. Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.
Clean thoroughly. This is especially important when using glass that is not formulated for fusing. Devitrification is more likely on these glasses. Water with a drop of dishwashing liquid can be enough unless your water has high mineral content. Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted. Finish with a polish to dry with clean paper towels. More here.
Firing the Slump
Fire up slowly. You should advance at about 100°C to 150°C per hour. Set your top temperature around 630°C for a simple slump, for soda lime stained glass. For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.
It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing. It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.
It is necessary to observe the progress of the slump as you do not yet know the slumping temperature. You want to know when the glass begins to deform so that you do not over fire. Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass. There is not much light in the kiln at these temperatures, so an external light is useful. You can also observe the reflections of the elements on the glass. When the image of the elements begins to curve, you know the glass is beginning to bend.
Altering the Schedule
Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough. Continue checking at 5-10 minute intervals to know when the slump is complete. If the glass is completely slumped before the soak time is finished, advance to the next segment. If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing. Consult your controller manual to learn how to do these things.
Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins. In some cases, you may need to also increase the temperature by 5-10°C.
The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point. It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.
The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes. The annealing cool can be as fast as 120°C down to 370°C. For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool. The annealing cool in this case could be about 60°C/hour to 370°C. You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C. The rate for the final cooling can be approximately double the first cooling rate. For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C