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GLASS BLOWING PROCEDURE
Cutting of glass tubing
- Make a short etched line at the point where the glass tubing is to be cut using a triangular file.
(i) To break the tubing at the etched mark, the tubing is held in both hands with the thumb placed on each side of the mark and close to the mark.
(ii) The glass is then pulled slightly towards its ends at the same time broken away from the body in one Motion.
NB: This procedure is only suitable for glassware which are less than 10mm in diameter. For tubing with over10mm diameter, the following procedures should be adopted.
- Etch the entire circumference at the desired line of cleavage, taking care that the two ends of the line meet in a complete circle.
- This scored line can also be tied with a piece of thread soaked in a suitable flammable solvent and then lighted.
- The glass cracks around the line and the two ends can be pulled apart.
- After glass tubing has been cut, the sharp edges should be flame polished in order to make them smooth.
Bending of glass tubing
(i) Formation of L – bends
L-bend glass tubing is used to convey liquids or gases from one container or flask to another .bends should be well rounded so that the tubing retains their original diameters. , A burner with a wide flame is generally used for the bending of the glass tubing. A tube is heated while holding it with both hands in length so that 5-8 cm area is heated while making sure that it is evenly and slowly rotated about its axis so as to heat all sides equally.
As soon as the glass is felt to be soft enough it’s slowly bent to the required shape. These is done by removing it from the flame and allowing one end to fall gradually under its own weight while being guided so that it is in the same plane as the rest of the tube.
The glass tubing must not be forced to bend otherwise a bend with a kink will be formed
Joining two straigt tubes or Butt Seals
Butt Sealing is the process of joining two sections of glass tubing together – end to end. This exercise demonstrates the joining of two similar size tubes together. A variation of this seal would be the joining of two dissimilar size tubes. Two options are offered – one requires good rotational skills.
1. Cut tubing into 8″ – 12″ lengths, fire polish ends.
- Attach blowhose assembly to one tube.
- Seal second tube with a cork or use a test tube bottom from the previous lesson.
- Light torch and adjust flame.
- Rotate ends of glass tubing (as if fire polishing) in flame.
6. When tube ends are red hot remove from flame, stop rotating, push tubes together making contact all around.
Another Option
This option does not require rotation. It does require the glassblower to hold and manipulate the torch around the glass tube that is held in a standard laboratory ring stand.
1. Clamp tube #1 vertically in a ring stand. Tube end about chin high with blow hose attached.
- Hold in one hand the torch adjusted to a sharp flame. A sealed tube (#2) is held in the other hand.
- Sweep flame back and forth across the end of tube #1, concentrating on the back half of the tube.
- Tube #2 is simultaneously held directly above tube #1 at a slight angle. This tube will also be heated as in step 3.
- Bring tube #2 down contacting the back of tube #1, continue to sweep flame back and forth across the tube ends.
- Continue to bring tube #2 down onto #1 making complete contact around the circumference. With practice steps 5 and 6 are completed in a quick smooth continuous motion.
- Using a small sharp flame, work out the seal by heating small sections of the seal at a time.
8.Anneal
(iii) Joining two bulbs of different diameters
The larger diameter tubing is sealed at one end with rubber tubing and the other end is rotated in the flame until the bore is the same diameter as that of the
smaller tube. One end of the smaller tube is then gently heated and the two hot ends are joined together. The flame is reduced and the junction is rotated over it.
Heat here
When red hot, the tubing is removed and the free end is gently blown to form a small bulb at the junction. The process is repeated until the two tubes are fused together, but when fusion is complete, the two tubes are pulled slightly apart to cause slight tempering at the junction.
( c) Blowing of bulbs
(i) End bulbs
Place the required end of the bulb on the flame and rotate until it becomes sealed .remove from the flame and blow while still hot through the other end until a small bulb appears.
End bulb
Using a large flame, reheat the bulb and a portion of the adjacent tube and gently blow again when it’s soft. End bulbs can be used to make thermometers.
(ii) Center bulbs
Stopper one end and heat the middle of the tubing until it’s soft and red. Remove it from the flame and press both ends towards the center to form a ring.
Heat the ring formed until its red-hot and blow. Repeat the process until the bulb formed reaches the required size.
Center bulb
T and Y joints (T seals)
T seals are simply two tubes sealed together at right angles. Modified T seals may have the glass tubes at any angle degree, the basic technique being the same. Joining dissimilar sized tubes end to end may be more appropriately classified as butt seals, but for our purposes are carried in this section because the technique shown is similar to the T seal.
1. Clamp a sealed 10 mm tube horizontally in a ring stand. Attach blowhose assembly to tube.
- Light and adjust torch to a sharp flame. Remove from holder.
- Heat a 10 mm spot on top of tube.
- Remove torch flame – blow out a bubble about 10 mm in diameter (do not pop).
- Reheat bubble and let glass collapse to a flat even plateau, remove torch. Sides of plateau should be about 3 mm high.
6. Reheat plateau, remove torch and blow a second bubble, (do not pop).
7. Position flame to cross over the top of the bubble, blowing hard at the same time to pop bubble. - Melt glass evenly down, forming a hole 10 mm in diameter with level 3 mm high side walls.