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Glass

Glass design

Rosenthal glasses are designed by renowned artists and designers. Glasses which are being designed for the Rosenthal studio-line, Rosenthal and Thomas are developed and reworked until the result is a matured range of stemware series and giftware items which are being passed on to the glassworks for production.

Raw materials
Depending on the composition of the glass required and the raw materials used, many types of glass with most varied characteristics can be melted. Rosenthal create their products from crystal glass or lead crystal. Crystal glass is a pure, hard colourless glass with a strong light refraction and a good resonance. It is produced from quartz sand, soda, potash and calcium with the addition of some barium and minium. Lead crystal is heavier and softer that crystal glass. It has a particularly strong light refraction and is therefore very suitable for cut decorations, which bring its visual qualities into full effect. It is mainly smelted using the same raw materials as those for crystal glass and contains 24% of lead oxide as determined by the EU-standard. The addition of metal oxides allows the production of glass in almost any colour.

Glass furnace

These days in hand blowing glass works crystal glass and lead crystal is melted in tank furnaces as well as in glasshouse pots. A pot furnace usually consists of 1 - 5 pots made from fireclay which are slowly preheated to above 1100 degrees C. The preheating prevents the pots from cracking when placed in the melting furnace. These pots can hold between 600 - 900 kg of glass. The life span of a brick pot is about 12 - 16 weeks. The furnaces are fired with gas and light oil and are used for several years without interruption. The walls of the glass furnaces have openings through which the glassblowers gather the molten glass from one of the pots with their special blowing irons. Whereas in pot furnaces glass is melted and worked with in a daily rhythm, tank furnaces can be used continuously. A glass level regulator automatically refills the amount of molten glass as it is extruded from the front. Production is carried out in shifts.

Glass smelting

The preparing, weighing and treatment of the raw materials for glass has to be carried out with great care and precision. The smallest variation in the quantities results in a change of glass quality. The purpose of glassmelting is to change the raw mixture into a homo- genousliquid and it is essential to bond the very finely ground raw ingredients. The smelting worker melts the glass in the pot furnaces overnight. After the glass batch has been fed into the furnace, the smelting process itself consists of three phases. The first phase produces a liquid glass full of bubbles, the raw smelt. During the second phase, the fine smelt, the temperature is increased to 1400 degrees C. Then the smelt worker "bubbles" the glass by dipping a piece of wet wood into the glass, causing the bubbles to rise to the surface. Here and there tiny bubbles can remain and will be visible later in the finished glass. In the third phase the glass has to cool slowly to a working temperature of 1200 degrees C. The next morning the glassmakers can start their work.

Glass blowing
With his blowing iron the glassblower gathers a small globule of molten glass and blows it into a round "gob". Dipping the blowing iron with the gob back into the pot, he gathers enough glass needed to produce the intended piece by rapidly turning the iron. By blowing, swinging and turning it in a wooden spoon, the article is then pre-shaped. In serial production of hand blown glass, it is blown into a wooden or steel mould. The glass maker lowers his blowing iron into the mould until, by constantly blowing and spinning, the glass is forced into the shape of the mould and has cooled and hardened. Then the mould is opened and the article removed.

Goblets

The first step in producing a goblet is the making of the bowl, as described above. Then a small knob of red-hot glass is gathered and attached in the centre of the bottom of the bowl with extreme precision. Using a simple tool the glassmaker then draws the knob into a stem, cutting off any excess glass. A second knob is then attached to the base of the stem, which the glassmaker quickly pats into shape with wooden clappers to form the foot. Where the glass has a "drawn" stem, a cone of glass is blown into the centre of the bottom of the glass. The cone is reheated and then drawn and further shaped like an attached stem. Forming the stem and foot requires a great deal of skill and a sharp eye. Years of practice and experience are needed to make a series of goblets, which are identical in appearance. A team of glassblowers usually consists of three to five craftsmen who must work hand in hand with speed and accuracy as glass hardens within only a few seconds. With hand blown glassware small variations of thickness in bowl, stem and foot cannot be avoided. The connoisseur appreciates this as distinguishing marks of hand blown stemware in comparison to machine-made glass.

Cooling
By the time a piece of stemware has assumed its final shape, it still has a temperature of about 450 degree C. It is removed from the blowing iron and transferred with a fork into the cooling oven where it gradually anneals to a normal temperature. This gradual cooling process is essential as it prevents the build up of stress within the glass, which would occur if the glass cooled too quickly. If a glass is not cooled or cooled too quickly, stress cracks can occur, even years later. This happens suddenly without contact. A typical tension-break runs smoothly, starting from the most solid part of the glass, e.g. the foot, or horizontally, approximately 2 cm below the glass rim.

Finishing the rim
Handblown glass has a "cap" where the blowpipe was attached to the upper part of the bowl. After cooling these caps are "cracked off" by first scoring a line around the bowl with a special wheel. An intense pinpoint gas flame heats the scored glass. By blowing air onto the glass, it cracks and the cap falls off. The glass now has its final shape. The very sharp rim of the bowl is then ground inside and out and finally finished off with a gas flame to make it smooth to the touch.

The finishing touch
A high degree of skill is needed for the finishing touch. For this purpose an iron rod or "punty" is attached to the foot of the completed, but still hot piece to hold it steady. Now the cap is cracked off with a sharp file. The unfinished rim is re-heated in the furnace so that the surplus glass can be removed smoothly with a pair of scissors. After further re-heating, the rim is adjusted to the desired shape and diameter. Now the punty is broken off, leaving a rough scar, the navel, which is then ground and polished after the glass is cooled. This mark is a distinguishing feature that tells the connoisseur the piece is hand blown and finished by hand.

Glasses with injected stems
A technique makes it possible to produce glasses with relief-patterned stems, which could not be achieved using the traditional methods. A steel mould is made which contains inside the negative image of the head relief. The hand blown bowl is placed on top of the mould and molten glass is forced into the mould from below with compressed air. Once the glass is cooled down and the head is safely attached to the bowl, the mould can be removed. After that the stem and foot are attached by hand.

Optically blown glass
A very old, yet a very fashionable technique of decorating glass in its hot state these days is "optical blowing". This method stretches the glassmakers ability to the full and requires a special dexterity and years of experience. To produce an optically blown glass article, the glob of molten glass at the end of the blowing iron is lowered into an open cylindrical mould and blown against the ribbed or patterned walls so that the pattern becomes imprinted in the glass. The still pliable glass is then blown into a finishing mould, retaining the imprinted pattern optically. The glassblower can achieve additional effects by twisting the fluted pattern into a spiral, turning the mould fast in one direction only.

Other glasswork decorations
There are many different ways for the glassmaker to decorate glass whilst it is still very hot and pliable. Air bubbles and lenses can be worked into the glass. Filaments of glass in contrasting colours can be overlaid on the molten glass, or the glassmaker can create an article from two or three different types of glass or from multi-coloured glass. By virtue of its inherent properties, glass lends itself to decoration of almost unlimited variety. Such techniques, however, demand the greatest dexterity and can be very labour intensive. It would take too long to describe all the techniques that were developed over centuries of glassmaking.

Pressing
Technology has made it possible to press glass and has opened the way to producing very complicated patterned glass articles which cannot be produced in any other way. Rosenthal use lead crystal for their exquisite pressed glass articles. Molten glass is pressed by means of a hand press and under high pressure into a multisectional steel mould. Once the glass has hardened, the article can be removed and cooled. In special cases the surplus glass, which is attached to the piece in the shape of a funnel, must be ground down and polished and the grind mark cleaned off after cooling. The pressed glass articles are finally acid polished to a very high gloss.

Automated glass production
Producing glass articles by machine has been improved to such an extent over the last years that it is possible now to mass-produce well shaped glass in very high quantities. Glass for fully automated production is melted in a continuous process in a tank furnace. Carefully measured amounts of glass are taken out of the tank furnace at regular intervals, which, after a quick pre-pressing, are then blown with compressed air into a blow mould that is continually turning. The finished bowl, once taken out of the mould is then taken to a second mould. Here the stem and foot are attached. Then the glass is taken to the cooling track. Cracking off the caps, polishing and melting the glass rim is also done in a fully automated process

Cut-glass process
Finished glass vessels are cut under running water with vertical carborundum grinding wheels or other very hard synthetic stone. These wheels have different profiles for the various patterns. There are various types of cut such as the surface or paring-cut for edgings i.e. for goblet stems; the rolling for creating balls and olives, or the square wedge and convex-grinding cut. The pattern is traced on to the glass and then "pre-scored" by the grinder with a coarse grain grinding wheel, followed by fine "cutting" with a finegrain wheel. The now silky smooth surface is polished to a high gloss with a wood-, cork- or brush-wheel and a polishing agent.

Acid polishing

Acid polishing is a more economical process than polishing by hand. Hydrofluoric acid is the only acid affecting glass. The cut glass is dipped into a bath of hydrofluoric and sulphuric acid and polished to a high gloss by stirring continuously for 20 - 25 minutes. This mixture removes all irregularities and gives the glass a smooth and shining surface. Acid polishing is an extremely complicated procedure since the acid composition has to match the composition of each glass.

Äcid etching

Unlike mechanical techniques of decorating glass, acid engraving uses chemicals to create a pattern. The glass is dipped briefly into a bath containing mainly hydrofluoric acid. Before the acid etching, the areas to remain smooth are coated with a protective varnish. The protective varnish is then removed, revealing a lightly etched pattern on the surface of the glass. Acid-etched glassware can also be gilded and polished, making extremely precious and highly prized items.

Pantographic decoration

With this method the finished glass is coated with a layer of wax or paraffin. Following a template the filigree decoration is engraved through the protective layer using a steel needle. The glass surface is exposed again. This is an automatic process. To finish it off the glass is being acid etched.

Engraving
Engraving is a perfect means for applying fine, delicate designs to glassware. The engraving process is very similar to the cut-glass process, except that the engravers tools are fitted with minute grinding wheels. Using combinations of finest lines and shadings, the engraver can "draw" any number of motifs e.g. coats of arms, writings, ornaments and figures to a design covering the entire surface. The motif is engraved freehand without any need for guidelines. It takes years of experience and practice.

Sandblasted designs
Sandblasting is a new technique used for decorating glass. Extensive and generous design effects can be achieved with sandblasting. The glass is covered with a special stencil, exposing only those areas which are to be decorated. A stream of compressed air forces a jet of fine sand from the gun, roughening and matting the chosen areas. Sandblasted designs are often combined with engraved lines. The company logo is normally sandblasted onto the glass.

Hand painting
Colour decorations can be painted by hand or applied by means of transfers in the same way as for porcelain. Gold and platinum lines are always hand painted. The decorations are fired onto the glass at a temperature of approximately 560 degrees C.

Sorting

All glasses are sorted before packaging. It is important to differentiate: Faults in the glass e.g. impurities, faults at production stage e.g. badly shaped glass rim and faults in the finish of the glass e.g. bad cut. Hand blown and handcrafted glasses will always have distinctive differences, which are not to be mistaken for quality flaws. They are a sign that the glass is hand-made. The smallest, finest and barely visible air bubbles (up to 0.5 mm) do no rate as quality flaws. They are material inherent. Insignificant and inconspicuous tolerances in the thickness of the goblet rims also rate as impeccable. Only obvious deviations or wobbly, lopsided or chipped glasses are rejected as unacceptable. Handcrafted items will always show variations within set standards.

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