Porcelain furnaces
Porcelain furnaces
Modern, 21st century porcelain furnaces are technically sophisticated, electronically-controlled devices with programmable cycles for firing dental porcelains. These include metal-ceramics for firing onto metal frameworks (classic precious or non-precious alloys, titanium) or all-ceramics such as zirconia or lithium disilicate. All-ceramic inlays or laminate veneers can be fired directly onto refractory model dies.
The principle unit of a porcelain furnace is its refractory firing chamber. Once the porcelain has been built up, the restorations can be placed onto mesh, cones, pins or firing pads for firing.
The heating coils are usually located in the upper housing of the furnace and arranged concentrically around the restoration. A motor-driven mechanism closes the firing chamber with the restoration inside, either by raising the firing platform or lowering the upper housing of the furnace. The firing cycle settings depend on the material being fired/procedures and run according to pre-set, standardised or custom programmes.
Many settings can be programmed precisely and independently of each other, for example times can be set to the split second (preheating/drying, heat-rate, hold-time, cooling) and firing temperatures for various materials such as opaquer, shoulder and dentine porcelains as well as glaze firings programmed accurately.
As the only way of preventing undesirable opacity in the porcelain is to evacuate the firing chamber during firing (vacuum phase), a built-in powerful vacuum pump is an essential part of a porcelain furnace.
Porcelain furnace
Combined firing/pressing furnaces are used for fabricating pressed-ceramic restorations (pressing procedure resembling casting which makes use of pressure and heat to liquefy ceramic blocks and force them into lost, refractory investment moulds) using special firing chambers and pressure plungers.
Whereas glass infiltration firing of presintered ceramic is possible in a porcelain furnace ("infiltration firing"), special high temperature sintering furnaces are required for the actual sintering process (such as for zirconia).
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| English | German |
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| palliative | krankheitslindernd |
Focus text of the month
Composite veneers (also called "hybrid veneers") are laminate veneers for natural teeth. They consist of composite/composite resins (resin matrix with porcelain filler particles). The first attempts… Composite veneers (also called "hybrid veneers") are laminate veneers for natural teeth. They consist of composite/composite resins (resin matrix with porcelain filler particles). The first attempts were undertaken in the 1930s (at that time without the adhesive technique). Composite veneers share various features with porcelain veneers: They are mainly used for aesthetic purposes. The appearance of a tooth is adapted to harmonise better with the other teeth. This includes, e.g. masking discolorations or structure anomalies of the tooth structure, correction of shape deviations (shortening, elongation, diminutive forms), malpositioning (rotation, tipping), addition of missing tooth structure (e.g. due to attrition or erosion) and closure of small gaps (diastemas). Another objective may be the temporary or permanent adjustment of the occlusal guidance (e.g. at the canines) and/or bite (bite raising). Preparation (often after preparatory stages such as mock-up, wax-up, fabrication of try-in veneers, templates, groove cutting) is minimally invasive; veneers are generally thin (minimum 0.3 mm to maximum approx. 1 mm). In contrast to a crown, large areas of the tooth structures are preserved. In anterior teeth veneers always cover the vestibular surface (with or without incisal edge coverage); in the posterior region they generally also cover part of or the entire occlusal surface. Following try-in (if necessary with try-in pastes for optical bridging of the cement gap) the veneer is usually luted adhesively using transparent or whitish, long-term, shade-stable composite cements. Unlike porcelain veneers, cost-effective composite veneers can also be fabricated directly intraorally (also for temporary restorations). They are not as hard and consequently less wear-resistant, but this also helps them adapt more easily to individual patient conditions. Composite veneers exhibit a higher tendency to plaque accumulation and discoloration. These material-related differences decrease with newer composite materials ("polymer porcelain"). Composite veneers can be repaired intraorally. In addition to fabrication (using the build-up technique) of composite veneers, sometimes directly on the tooth but mainly custom-fabricated indirectly in the laboratory after impression-taking and model fabrication, systems with prefabricated composite veneers are available. These systems comprise a range of "blanks" for reproducing several versions of the individual tooth types. After respective pre-selection and minimally invasive preparation or without any preparation ("non-prep veneers"), these industrially manufactured homogeneous, surface-hardened composite veneers are customised by trimming and the addition of special composite materials before being luted adhesively. The service life of composite veneers should exceed six years. |