EXPERIMENT 13 GLASSWORKING OPERATIONS USING BUNSENIBATWING BURNER Structure 13.1 Introduction ObJectivea 13.2 Cutting Glass Rod and Tubing Requirements Procedure 1 for RocUTubing under 8 mrn 0.d. Procedure 2 far RodtTubing over 8 mrn 0.d. 13.3 Flame Polishing Requirements Procedure 13.4 Use of Carbon Blocks Requiremen@ Procedure 13.5 Blowing Bulbs Requirements Prosedure 13.6 Bending Narrow Bore Tubing Requirevents Procedure for forming a Bend using aQ Ordinary flame Procedure fgr forming a Bend using a Wide Flame Procedure for forming a U-Bend 13.7 Pulling Points in Glass Tubing Requirements Procedure for Pulling a Point in a Glasq Tubing Procedure for Pulling a Point at the End of a Glass Tubing 13.8 Joining Glass Tubings of Equal Diametel: ~e~u,irements Procedure 13.9 Blawing a Bulb in the Middle of a Tube Requirements Procedure 13.10 Summary 13. INTRODUCTION - " In Units 4 and 5 of Block 2, you studied different types of glass and their characteristics, basic tools needed for glassworking, techniques of glassworking, possible hazards from glass and first aid treatment for accidents from glwsworking. In this experi&nt, you will perform certain glassblowing operations with soda glass by using B-msenfbatwingburner. In the next experiment, you will perform these operations with borosilicate glass by using premixing type burners. In these experiments, the techniques which you will be expected to master are arranged in a logical progression with eadier techniques being the foundations for later work. Therefore, you should not be tempted to rush at the exercises or attempt to progress through them at too fast a rate. Just work through them steadily, taking long enough over each exercise to truly master the manipulations involved. Glasswork is an art, and only repeated practice I Basic Experiments will bring skill. If you are working in a laboratory, you can complete most of I in Chemktry I the glassworking operations involved in this experiment at your work place. In case, you have any difficulty in completing the work, try to do the same in I laboratory sessions at your study centre. Objectives After performing this experiment, you should be able to: cut soda glass rod or tubing, flame polish soda glass tubing and rod, blow bulb in a soda glass tubing, bend narrow bore soda glass tubing, pull points in a soda glass tubing, and join soda glass tubings of small and equal diameter.. 13.2 CUTTING SODA GLASS RODITUBING The unpredictable way in which glass fractures is discussed in Unit 4. This unpredictability is equally true of rod and tubing and in order to produce the right kind of break, the crack formed must be led in the required direction. Although the same principle is involved, two different techniques are used for two different ranges of size of rod or tubing. .The two size ranges are up to 8 mm outside/overall diameter (0.d.) and over 8 mm 0.d. 13.2.1 Requirements Glass rod or tubing of various diameters and lengths A V-stand A cloth A glass cutting knife or file 13.2.2 Procedure 1 for RodITubing under 8 mm 0.d. (1) Take a clean and flawless length of glass rod (or tubing) and place it flat on your work-bench. (2) Measure the length of rod required and mark it for cutting, (3) With the rod supported to prevent it from rolling on the work-bench, cut a nick in the rod using your glass cutter. The nick is cut by drawing the cutter quickly across the rod once only- see Fig. 13.1. The effect on the Fig.13.1: Cutting a nick in glass rod. Experiment 13 rod is shown in Fig. 13.2. \ Fig.13.2: A nick. The nick is deep but is sharply cleft so that a crack can lead directly from the bottom of the V. In fact, heat is developed in the nicking which initiates the crack from the source. (4) DO NOT cut a nick in the rod by pushing the cutter against the rod and supporting the rod with the thumb as illustrated in Fig. 13.3. Although you won't cut yourself with the cutter, the glass may break prematurely wd you stand a good chance of embedding glass in your thumb. Fig.13.3: Incorreet method for cuttiag a nick. (5) If the rod you are cutting is long, e.g. 2 mysupport the end farthest from the cut in a V-stand - see Fig. 13.4. This will prevent any whipping action in the long part of the tube in step (8) which will cause it to break into two or three pieces. Fig.13.4: Supporting a length of rod during breaking. Basic Experiments (6) Make sure that you know where the nick is and which way it is f~ing; in Chemistry and loosely wrap the area of the nick with the cloth. You will need to know the'exact location and orientation of the nick in the next step. (7) Hold the rod horizontally in front of you with both hands. With the nick facing away from you and a hand placed either side of the nick and shielded by the cloth, place your thumbs together on the rod opposite the nick. The correct position is shown in Fig. 13.5 where the cloth has been omitted for clarity. Fig.13.5: Hand position f~rbreaking a rod (safety cloth omitted for clarity). (8) Apply pressure with the thumbs and fingers in the directions indicated SAFETY by arrows in Fig. 13.5. This breaking action produces symmetrical , WARNING: When glass rod forces about the nick which run across the tube giving a right-angled or tube is cut, two main break. hazards arise: (a) The cutting process forms (9) DO NOT pull either part of the rod to one side, otherwise one cut end minute fragments and will include sharp tongued edges. splinters of glass which may cut or penetrate the To overcome these hazards: skim. Avoid contact. (b) The cut ends of rod and (1) Always maintain an awareness of these hazards. tube fonn dangerous cutting edges which must (2) Keep the working area clean. never be handled or touched. (3) Only dispose of cold glass waste into metal waste bins specially marked for this purpose. (4) Avoid brushing working surfaces with your hands. (5) Always remove any tongues from newly cut surfaces. You should do this by 'filing' them off with a wire gauze - see Fig. 13.6. Hold the gauze at a slight angle and move it down quite sharply in the direction illustrated in Fig. 13.6. The wires rattle over and remove the sharp tongue quite satisfactorily. Experiment 13 Fig.13.6: Removing tongues with wire gauze. (6) Flame polish cut ends as soon as practicable. 13.2.3 Procedure 2 for Rodmubing over 8 mm 0.d. Cutting wider tubing (or rod) is more difficult since you must score the glass for all its circumference. When cutting small tubing, the forces generated are roughly symmetrical because they are large in relation to the small distances. However, with tubing of, say 10 mm 0.d. and above, the forces lose symmetry over the greater distances causing a bad break. In order to avoid this, a crack has to be led all the way around when cutting a larger piece of tube. I (1) Carry out steps (1) and (2) of Procedure 1. (2) If you are using a glass knife, as in Procedure 1, with one hand hold the knife firmly against the glass, and with the other hand rotate the rod axially - see Fig. 13.7. The resulting score should meet itself after one revolution of the rod to form a complete ring. Fig.13.7: Scoring a ring round wide tubing. (3) An easier method is to use a diamond tipped, wide tubing cutter. Place the glass in the .wide V in the tool - see Fig.13.8, and position the diamond tip (mounted on the hinged arm) on the side of the glass opposite the V support. Holding the cutting arm firmly with light pressure only on the glass rod with one hand, you should turn the glass rod axially with your other hand until a complete ring has been inscribed. Buk Experiments In steps (2) and (3), ensure that you do not turn the glass for more than in Chemistry one revolution. Fig.13.8: Diamond tipped, wide tubing cutter. You should now start the crack, for which you can use one of the three methods described in steps (6), (7) and (8). Hot spotting is a technique using a 2 rnnl diameter glass rod (pulled from a piece of scrap rod) the tip of which is heated. (a) Heat the tip of the 2 mm rod in a flame until it forms a small, molten blob. (b) While the tip of the rod is still molten apply it to the ring scored in steps (2) or (3) - see Fig. 13.9. Fig.13.9: Leading a crack by hot spotting. (c) The hot spot will start a crack which may progress all round the tube. If it doesn't, reapply the reheated hot spot to any uncracked areas until the crack is complete. Another technique is to use heated wire loops - see Fig. 1 3.10. The loops are made of steel or iron wire, in the form of a hook and with one end mounted in a wooden handle. Different sized loops are required for tubes of different diameters.