GLASS BLOWING TECHNOLOGY DOMINIC MAGUT NAIROBI TECHNICAL TRAINING

GLASS BLOWING TECHNOLOGY DOMINIC MAGUT NAIROBI TECHNICAL TRAINING INSTITUTE

OBJECTIVES In this chapter, we will Define the term glassblowing Discuss importance of glassblowing in the laboratory Describe composition and properties of glass used for glassblowing Describe various tools used for glassblowing Describe glass blowing techniques used in making simple glass apparatus Discuss various conditions encountered in glass blowing Discuss how to correct conditions encountered in glass blowing

INTRO. Glassblowing is a glass forming technique that involve inflating, cutting or shaping molten glass tubbing into bulbs and other different shapes to serve as glass apparatus. Glass is a product formed by fusion of various oxides, which have been cooled down to rigid conditions by special freezing technique whereby the liquid glass cools down without crystallization.

CLASSIFICATION OF GLASS Glass is classified according to the parent oxide from which they were formed. There are two types of glasses Soft glass - Soda lime glass or soda glass Hard glass – resistant glass or borosilicate glass

Major characteristic and physical properties of glass

Effects of heat on glass Glass is a poor conductor of heat, when it is heated; the surface that is near to the flame undergoes compression strains while the inner surface undergoes tension strains. When it is removed from the flame, the air around it affects the outside layer fat and it therefore undergoes tensile strength while the inner layer undergoes compression strains. I. e. the outer layer cools faster than the inner layer and hence enforces a strain within the inner layer, these causes cracking. To remove strains from glass, the glass is normally put into an annealing oven at a certain temperature (annealing temperature) and left for some time. Eventually the temperature is reduced stepwise at certain intervals until room temperature is reached. The starting temperature should always be 5 oc higher than the annealing temperature.

Properties of Glass (a) Soda glass or soft glass Soda glass melts on a Bunsen burner flame. They are prone to diventrification (forms crystals easily ) therefore they need careful introduction on a flame. They are very cheap and readily available in any quantity and size. Give an intense and persistence yellow sodium flame on heating. They are smooth on feeling with finger i. e. the edges will feel smooth on scratching. When viewed against a source of light , its edge will have a green tinge.

Properties of Glass cont. (b) Borosilicate glass is a hard colorless glass with a low coefficient of expansion it softens at high temperatures Can be put to the flame directly and requires less annealing than soda glass They are rough on feeling or the edge will feel rough on scratching It’s expensive than soda glass

Relative chemical durability Refers to the amount of glass removed per square meter surface area of glass after exposure to a chemical or reagent (e. g. Na. OH, HCl etc. ) in 24 hours at 95 oc. Soda glass loses glass in Na. OH because Na+ move to the surface of the glass. Strong alkali solution attack glass therefore they should not be stored in glass. Acidic solution except phosphoric acid and hydrofluoric acid do not attack glass , hence they can be stored in glass containers

Glass blowing facilities and equipment A glass-blowing can be done in the laboratory. The required equipment includes a furnace, glory an hole, annealer, a bench, various tools to blow and shape the glass.

Glass blowing facilities and equipment Space The proper equipment for glass blowing takes up a lot of room. Your workspace will need to be large enough to accommodate not one, but three separate furnaces (a large furnace, glory, and an annealer), in addition to a workbench, a marvel, and the various tools of the trade. Safety should always be your top priority You must be confident in your skills, and always try to have at least one other person in the room with you when you’re working so that in the event that something goes wrong, help is available immediately.

Glass blowing facilities and equipment Cont. Furnaces To operate a modern glass-blowing operation, you will first need a large furnace capable of reaching temperatures exceeding 2, 000℉. In addition to the furnace, you’ll also need what is known as a glory hole or a reheating furnace. This is a smaller furnace, usually cylindrical in shape, with a small hole used to reheat glass on the end of the blowpipe to keep it malleable so that it can continue to be worked and shaped. Glory holes are typically heated to around 1, 500℉, sometimes more, to enable the glass blower to quickly reheat the piece and continue working.

Glass blowing facilities and equipment Cont. Annealer An annealing oven is used to safely cool the glass. This piece of equipment is necessary to relieve the stress on the glass and to prevent shattering. When the glass piece is first finished, it is somewhere around 1, 400 – 1, 500℉, but some areas will be cooler, and some might be hotter. An annealer brings the entire thickness of the piece to the same temperature, usually around 1, 000℉. It then slowly cools the glass down over a period of 12 – 14 hours, though times can vary based on the item’s thickness and type of glass used.

Glass blowing facilities and equipment Cont. Blow pipe A blow pipe is a hollow steel tube used for collecting, holding, and blowing into the glass. The blowpipe must be kept in constant motion to prevent the force of gravity from misshaping your molten glass.

Glass blowing facilities and equipment Cont. Other tools include ; A Marver – metal table for shaping glass. Diamond glass cutter – used for cutting glass Triangular file – used for polishing the ends of the glass tubing , for making a nick or to effect a mark on a glass tubing Eye goggles – used for the safety of the eyes A Bench – provides seating for the worker, a place to rest tools, and rails to support the pipe while being rolled. Jacks – to shape side walls of the glass.

Glass blowing facilities and equipment Cont. Tweezers – used to hold the glass when transferring it to the punty. Punty – metal rod attached to the bottom of the glass to free up the top portion Crimp – tool used to add decorative elements Paddles – wood boards used to flatten sections of glass. Different size blocks – wood tools soaked in water that produce steam when they contact the hot glass, allowing the glass to slide easily while shaping. Gas cylinder – used for storage of gas

Glass blowing facilities and equipment Cont. Flaring tool – used for welding glass tubing Carbon plate -used for flattening the bottom of glass container burners - used for heating and also for flame polishing Annealing oven - used for removing strains in glass Trimmer used to effect the break of a large diameter tubing NB ; flame polishing is the process of smoothening a glass rod after cutting

GENERAL PROCEDURE FOR MAKING GLASS Before starting the glass blowing process, the glass is placed in a furnace that heats it to a temperature of 2000 degrees, making it malleable. Next, the glass is gathered by inserting one end of the blowpipe into the furnace, and rolling it over the molten glass until a “gob” of glass attaches to it. The next step is to roll the molten glass on a flat metal slab called a marver. The marver acts as a means to control the shape and temperature of the glass. The glass is taken back and forth from the marver to the glory hole, a hot chamber used to reheat the glass in order to make it malleable again.

GENERAL PROCEDURE FOR MAKING GLASS cont. To give the glass color and design, it’s dipped in crushed colored glass, which fuses to the main glass piece almost immediately due to the hot temperature. Once the main glass piece has been fused with crushed colored glass, it is taken back to the marvel where it is rolled again. To give the glass its final shape and size, it is blown into with a blowpipe, creating a sort of bubble of glass. To carry out this process, the blowpipe holding the glass must be placed on a steel stand. Then, the glass artist has to blow into the blowpipe while rotating it at the same time. Throughout this process, the glass needs to be continuously taken to the glory hole to be reheated because blowing it cools it very quickly.

GENERAL PROCEDURE FOR MAKING GLASS cont. The final step is to remove the glass from the glass pipe. To do this, steel tweezers called jacks are used to separate the bottom part of the blown glass while rotating the blowpipe. The last step is to take the blown glass to an annealing oven using heat resistant gloves. This allows the glass to cool slowly over several hours, as it is highly perceptive to breaking when exposed to rapid temperature changes.

Cutting of glass tubing (i) 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. 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 10 mm in diameter.

Cutting of glass tubing cont. For tubing with over 10 mm 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. These 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 have 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 are 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 its evenly and slowly rotated about its axis so as to heat all sides equally.

Bending of glass tubing cont. 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 kink Bending of glass tubing with larger diameters requires first closing one of its end with a stopper, then a wide section of it is rotated over a burner with a wide flame, as soon as the glass softens , its removed from the flame and bent upwards, while at the same time gently blowing into the free end.

Joining two tubing of the same diameter To join two tubing together, one end of one tube is stoppered and one end of each tubing is gently heated. They are then both removed from the flame and the two heated ends are slightly pressed into each other. The flame is reduced, and the junction is carefully rotated over it. When red hot, the tubing is removed from the flame and the free end gently blown to form a small bulb at the junction The process is repeated until when the two tubing are fussed together and the line of junction has disappeared. It’s then bowed and gently cooled until the bore is of uniform diameter

Joining two bulbs of different diameters The larger diameter tubing is sealed at one end with a 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. 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.

Blowing of bulbs a. End bulb 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.

Blowing of bulbs b. 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

Making T and Y bends Seal one end of a piece of glass tubing , hold the tube steadily and soften a small area with a very small flame. Remove the tubing from the flame and blow gently until a small bulb appears. Repeat these operation until the diameter of the shoulder of the bulb is just slightly smaller than that of the tubing to be attached. Break the bulb by gently tapping so that only the shoulder remains Seal one end of the tubing to be attached and heat the other end while heating the shoulder of the bulb at the same time , when both ends are red hot , press them together and pull slightly apart while blowing gently down the unstoppered end

Making T and Y bends cont. Using a small flame , heat one side of the junction and blow gently into the unstoppered arm. repeat the process all round the circumference until the two tubes are fused together and the line of junction disappears. The Y tube is made in a similar way but the tube is first bent to the desired angle. The other bulb is then attached in the manner described above. All such junctions should be carefully annealed. T and Y joints are used for sharing services in the laboratory e. g. gas and water.

Annealing is the process of removal of strains from a fixed glass product it is a process of cooling down of the glass from its annealing temperature in a slow, gradual and stepwise manner until the room temperature is reached.

Bloom A bloom is a ring formed on a glass product about 3 mm from the point of heating because of sodium ions in the glass coming into contact with sulfur ions in the glass heating There are two types of blooms (a)Temporary bloom These can be wiped out on cooling (b)Permanent bloom This is only removed by chemical means Bloom

Devitrification of glass Devitrification is the process of formation of crystals in glass (crystallization of glass). Devitrified glass looks or appears translucent; it is rough and cannot be seen through easily. Its cause by; (i) Heating of a piece of glass on a hot flame for a very long time. (ii) Heating a piece of glass and then suddenly introducing it to a cool condition cooling of glass) (rapid (iii) Chemical weathering (due to old age ) , the moisture collects on glass and when it combines with atmospheric CO 2, it forms carbonic acid which eats up the soft sodium ions of the glass and the hard ions becomes crystallized or crystallizing on the surface of the glass

Devitrification can be eradicated by: Pouring strong solution of Na. OH on the glass surface and leaving it on for some time. These will etch up the ends of the glass that are sticking out and a smooth glass will be produced. NB this process may however result in the formation of thin glass. By fusion – in these case the piece of glass is put in an oven until it softens out so that the crystals ticking out of the glass will be fussed into the walls of the glass. These method is not suitable for volumetric glassware , because when they are heated to high temperatures , they don’t regain their original size on cooling.