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Round-Bottom and Florence Flasks Blown from Glass Tube
A round-bottom flask is a glass sphere on a tubular neck: the round shape heats evenly and stands up to vacuum, so it is the flask for boiling and distilling. Blow it from the end of a borosilicate tube; flatten the bottom while soft and it becomes a Florence flask that stands on the bench.
The picture: our diagram of the steps, from tube to round-bottom and Florence flask.
Advanced
An hour per flask, plus annealing
Instructions
1
1
Close and gather
Close and gather
Close one end of the tube in the flame as for a test tube. Keep heating and turning the closed end, pushing gently, so glass gathers into a thick knob for the bulb.
Materials for this step:
Glass Tubing Kit1 pieceTools needed:
Bench Glassblowing Torch
Safety Glasses
Heat-Resistant Gloves2
2
Blow the bulb
Blow the bulb
Take the gather out of the flame and blow gently into the open end while turning, so the bulb grows round. Reheat and blow again until it is the size you want.
Tools needed:
Bench Glassblowing Torch
Digital Caliper 6-InchSafety Glasses3
3
Flat bottom for a Florence flask
Flat bottom for a Florence flask
For a flask that stands, soften the bottom of the bulb and press it gently on a flat plate.
Tools needed:
Bench Glassblowing TorchHeat-Resistant Gloves4
4
Cut and finish the neck
Cut and finish the neck
Score and snap the tube to neck length, fire the edge smooth and flare it a little with the jacks so a stopper or joint fits.
Tools needed:
Diamond ScriberBench Glassblowing Torch
Jacks5
5
Anneal and check
Anneal and check
Anneal in the kiln at the glass's annealing temperature and cool slowly. Fill with measured water to find and mark the volume. Stand round flasks in a cork ring.
Materials for this step:
Water1 literTools needed:
Kiln
Measuring CylinderDiamond Scriber6
6
Bulb size for a volume
Bulb size for a volume
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7
7
Limits
Limits
The flask's form and why it is round are from Wikipedia; the blowing is our lampwork practice. Heat only borosilicate flasks, and never boil in a flask with a scratched or unannealed wall.
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