Lost-wax casting

Lost-wax casting – also called investment casting, precision casting, or cire perdue (French: [siʁ pɛʁdy]; borrowed from French)^[1] – is the process by which a duplicate sculpture (often a metal, such as silver, gold, brass, or bronze) is cast from an original sculpture. Intricate works can be achieved by this method.

The oldest known examples of this technique are approximately 6,500-year-old (4550–4450 BC) and attributed to gold artefacts found at Bulgaria's

The steps used in casting small bronze sculptures are fairly standardized, though the process today varies from

Casts can be made of the wax model itself, the direct method, or of a wax copy of a model that need not be of wax, the indirect method. These are the steps for the indirect process (the direct method starts at step 7):

1. Model-making. An artist or mould-maker creates an original model from wax, clay, or another material. Wax and oil-based clay are often preferred because these materials retain their softness.
2. Mouldmaking. A
   shim
   with keys is placed between the parts during construction so that the mould can be put back together accurately. If there are long, thin pieces extending out of the model, they are often cut off of the original and moulded separately. Sometimes many moulds are needed to recreate the original model, especially for large models.
3. Wax. Once the mould is finished, molten wax is poured into it and swished around until an even coating, usually about 3 mm (1⁄8 inch) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is to fill the entire mould with molten wax and let it cool until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.
4. Removal of wax. This hollow wax copy of the original model is removed from the mould. The model-maker may reuse the mould to make multiple copies, limited only by the durability of the mould.
5. Chasing. Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the
   flashing
   where the pieces of the mould came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were moulded separately can now be heated and attached; foundries often use registration marks to indicate exactly where they go.
6. Spruing. The wax copy is
   sprued
   with a treelike structure of wax that will eventually provide paths for the molten casting material to flow and for air to escape. The carefully planned spruing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy. The spruing does not have to be hollow, as it will be melted out later in the process.
7. Slurry. A sprued wax copy is dipped into a slurry of silica, then into a sand-like stucco, or dry crystalline silica of a controlled grain size. The slurry and grit combination is called ceramic shell mould material, although it is not literally made of ceramic. This shell is allowed to dry, and the process is repeated until at least a half-inch coating covers the entire piece. The bigger the piece, the thicker the shell needs to be. Only the inside of the cup is not coated, and the cup's flat top serves as the base upon which the piece stands during this process. The core is also filled with fire-proof material.
8. Burnout. The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out. The melted wax can be recovered and reused, although it is often simply burned up. Now all that remains of the original artwork is the negative space formerly occupied by the wax, inside the hardened ceramic shell. The feeder, vent tubes and cup are also now hollow.
9. Testing. The ceramic shell is allowed to cool, then is tested to see if water will flow freely through the feeder and vent tubes. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, then patched.
10. Pouring. The shell is reheated in the kiln to harden the patches and remove all traces of moisture, then placed cup-upward into a tub filled with sand. Metal is melted in a crucible in a furnace, then poured carefully into the shell. The shell has to be hot because otherwise the temperature difference would shatter it. The filled shells are then allowed to cool.
11. Release. The shell is hammered or sand-blasted away, releasing the rough casting. The sprues, which are also faithfully recreated in metal, are cut off, the material to be reused in another casting.
12. Metal-chasing. Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, so that the casting now looks like the original model. Pits left by air bubbles in the casting and the stubs of the spruing are filed down and polished.

Prior to silica-based casting moulds, these moulds were made of a variety of other fire-proof materials, the most common being plaster based, with added grout, and clay based. Prior to rubber moulds gelatine was used.

• 
  Step 1: A model of an apple in wax
• 
  Step 2: From the model a rubber mould is made. (The mould is shown here with a solid cast in plaster)
• 
  Step 3: From this rubber mould a hollow wax or paraffin cast is made
• 
  Step 4: The hollow paraffin apple is covered with a final, fire-proof mould, in this case clay-based, an open view. The core is also filled with fire-proof material. Note the stainless steel core supports. In the next step (not shown), the mould is heated in an oven upside-down and the wax is "lost"
• 
  Step 5: Liquid bronze at 1200°C is poured into the dried and empty casting mould
• 
  Step 6: the bronze cast, still with spruing attached. The sprue will be cut away and the final shape polished

Jewellery and small parts

The methods used for small parts and

The lost-wax process can be used with any material that can

The lost-wax casting process may also be used in the production of cast glass sculptures. The original sculpture is made from wax. The sculpture is then covered with mold material (e.g., plaster), except for the bottom of the mold which must remain open. When the mold has hardened, the encased sculpture is removed by applying heat to the bottom of the mold. This melts out the wax (the wax is 'lost') and destroys the original sculpture. The mold is then placed in a kiln upside down with a funnel-like cup on top that holds small chunks of glass. When the kiln is brought up to temperature (1450-1530 degrees Fahrenheit), the glass chunks melt and flow down into the mold. Annealing time is usually 3–5 days, and total kiln time is 5 or more days. After the mold is removed from the kiln, the mold material is removed to reveal the sculpture inside.

Archaeological history

Black Sea

Cast gold knucklebones, beads, and bracelets, found in graves at Bulgaria's Varna Necropolis, have been dated to approximately 6500 years BP. They are believed to be both some of the oldest known manufactured golden objects, and the oldest objects known to have been made using lost wax casting.^[2]

Middle East

Some of the oldest known examples of the lost-wax technique are the objects discovered in the Nahal Mishmar hoard in southern Land of Israel, and which belong to the Chalcolithic period (4500–3500 BC). Conservative Carbon-14 estimates date the items to around 3700 BC, making them more than 5700 years old.^[4][5]

Near East

In

The oldest known example of applying the lost-wax technique to copper casting comes from a 6,000-year-old (c. 4000 BC) copper, wheel-shaped amulet found at Mehrgarh, Pakistan.^[3]

Metal casting, by the

Gupta and post-Gupta period bronze figures have been recovered from the following sites:

The inhabitants of

Cast bronzes are known to have been produced in

Egypt

The

Direct imitations and local derivations of

Some Late Bronze Age sites in Cyprus have produced cast bronze figures of humans and animals. One example is the male figure found at Enkomi. Three objects from Cyprus (held in the Metropolitan Museum of Art in New York) were cast by the lost-wax technique from the 13th and 12th centuries BC, namely, the amphorae rim, the rod tripod, and the cast tripod.^[34]

Other, earlier examples that show this assembly of lost-wax cast pieces include the bronze head of the Chatsworth Apollo and the bronze head of Aphrodite from Satala (Turkey) from the British Museum.^[35]

East Asia

There is great variability in the use of the lost-wax method in East Asia. The casting method to make bronzes till the early phase of Eastern Zhou (770-256 BCE) was almost invariably section-mold process.^[36] Starting from around 600 BCE, there was an unmistakable rise of lost-wax casting in the central plains of China, first witnessed in the Chu cultural sphere.^[37] Further investigations have revealed this not to be the case as it is clear that the piece-mould casting method was the principal technique used to manufacture bronze vessels in China.^[38] The lost-wax technique did not appear in northern China until the 6th century BC.^[19] Lost-wax casting is known as rōgata in Japanese, and dates back to the Yayoi period, c. 200 BC.^[16] The most famous piece made by cire perdue is the bronze image of Buddha in the temple of the Todaiji monastery at Nara.^[16] It was made in sections between 743 and 749, allegedly using seven tons of wax.^[16]

Northern Europe

The Dunaverney (1050–910 BC) and Little Thetford (1000–701 BC) flesh-hooks have been shown to be made using a lost-wax process. The Little Thetford flesh-hook, in particular, employed distinctly inventive construction methods.^[39][40] The intricate Gloucester Candlestick (1104–1113 AD) was made as a single-piece wax model, then given a complex system of gates and vents before being invested in a mould.^[9]

Americas

The lost-wax casting tradition was developed by the peoples of

Some early

Many bronze statues or parts of statues in antiquity were cast using the lost wax process. Theodorus of Samos is commonly associated with bronze casting.^[46][49] Pliny also mentions the use of lead, which is known to help molten bronze flow into all areas and parts of complex moulds.^[50] Quintilian documents the casting of statues in parts, whose moulds may have been produced by the lost wax process. Scenes on the early-5th century BC Berlin Foundry Cup depict the creation of bronze statuary working, probably by the indirect method of lost-wax casting.^[51]

Direct evidence

India

The lost-wax method is well documented in ancient Indian literary sources. The Shilpa Shastras, a text from the Gupta Period (c. 320–550 AD), contains detailed information about casting images in metal. The 5th-century AD Vishnusamhita, an appendix to the Vishnu Purana, refers directly to the modeling of wax for making metal objects in chapter XIV: "if an image is to be made of metal, it must first be made of wax."^[15] Chapter 68 of the ancient Sanskrit text Mānasāra Silpa details casting idols in wax and is entitled Maduchchhista Vidhānam, or the "lost wax method".^[15][16] The 12th century text Mānasollāsa, allegedly written by King Someshvara III of the Western Chalukya Empire, also provides detail about lost-wax and other casting processes.^[15][16]

In a 16th-century treatise, the Uttarabhaga of the Śilparatna written by Srïkumāra, verses 32 to 52 of Chapter 2 ("Linga Lakshanam"), give detailed instructions on making a hollow casting.^[15][16]

Theophilus

An early medieval writer

• 
  A wax model is sprued with vents for casting metal and for the release of air, and covered in heat-resistant material.
• 
  A cast in bronze, still with spruing
• 
  A bronze cast, with part of the spruing cut away
• 
  A nearly finished bronze casting. Only the core supports have yet to be removed and closed
• 
  Hugo Rheinhold's Affe mit Schädel is cast out of bronze using the lost-wax process.

• 
  This bronze piece entitled Lazy Lady, by the sculptor Rowan Gillespie was cast using the lost-wax process.
• 
  The Blätterbrunnen of 1976 by Emil Cimiotti, as seen 2014 in the city center of

  Hanover, Germany

  . A lost-wax method was used for the bronze leaves.

See also

• Fusible core injection molding

References

Sources

• Forbes, R.J. (1971). Metallurgy in Antiquity, Part 1: Early metallurgy, the smith and his tools, gold, silver, and lead, zinc, and bronze. Studies in Ancient Technology. Vol. 8. Brill.
  ISBN 978-90-04-02652-0
  .
• Hart, G.H.; Keeley, G. (1945). Metal Work for Craftsmen. London, UK: Sir Isaac Pitman & Sons.
• Hodges, H. (1995) [1864]. Artifacts. Bloomsbury Academic.
  ISBN 978-0-7156-2316-9
  .
• Jones, D.M., ed. (2001). Archaeometallurgy. London, UK: English Heritage.
• Konkova, L.V.; Korol, G.G. "South Siberian imports in eastern Europe in the 10th—13th centuries: Traditions of metalworking". In Archaeometallurgy in Europe (2003).
• Long, S. (October 1964). "Cire Perdue Copper Casting in Pre-Columbian Mexico: An Experimental Approach". American Antiquity. 30 (2): 189–192.
  S2CID 163771730
  .
• McArthur, M. (2005). The Arts of Asia: Materials, techniques, styles. London, UK: Thames & Hudson.
• Maddin, Robert, ed. (21–26 October 1986). The beginning of the Use of Metals and Alloys. Second International Conference on the Beginning of the Use of Metals and Alloys. Zhengzhou, China: MIT Press (published 1988).
  OCLC 644557973
  .
• Maddin, Robert (1988). The Beginning of the Use of Metals and Alloys. The MIT Press.
  ISBN 9780262132329
  .
• Noble, J.V. (October 1975). "The wax of the lost wax process". American Journal of Archaeology. 79 (4): 368–9.
  S2CID 193070936
  .
• Peng, Peng (2020). Metalworking in Bronze Age China: The Lost-Wax Process. Cambria Press.
  ISBN 9781604979626
  .
• Taylor, S.E. (1978). Dark-Age metal casting: An experimental investigation into the possibility of using wax models for the formation of clay-piece moulds, with special reference to the manufacture of pairs of cast objects (Report). The Department of Archaeology Cardiff. Vol. 97. University of Cardiff.
• Trench, Lucy (2000). Materials & Techniques in the Decorative Arts: An illustrated dictionary. University of Chicago Press.
  ISBN 978-0-226-81200-7
  – via Internet Archive (archive.org).
• Proceedings. Archaeometallurgy in Europe: International Conference. Milan, IT: Associazione Italiana di Metallurgia. 24–26 September 2003.
  ISBN 978-88-85298-50-7
  .

External links

Wikimedia Commons has media related to Lost-wax casting.

External videos
Bronze casting (direct method). Khan Academy (video). Smarthistory. Retrieved 6 January 2013.
Adriaen de Vries's bronze casting techniques. J. Paul Getty Museum (video). Retrieved 22 January 2013.

• "The bronze casting process". Andre Stead Sculpture. Archived from the original on 6 November 2016.
• "Metal Art of Bastar Photos". chhattisgarh.blogspot.com (blog). February 2011.
• "Flash animation of lost-wax casting process". James Peniston Sculpture. Retrieved 24 October 2007.
• "Virtual foundry". National Museum of Wildlife Art. Archived from the original on 16 May 2008.
• "Casting a Medal". Sculpture. Victoria and Albert Museum. Archived from the original on 29 January 2009. Retrieved 22 September 2007.
• "Reconstructing the Bronze Age Trundholm Sun Chariot". comhem.se. Viking bronze. Archived from the original on 11 October 2008.
• "The "cire-perdue" process of bronze casting". Scientific American past. 15 October 1904. Archived from the original on 5 October 2011.
• Harvey, A. "André Harvey lost wax process (cire perdue)". Archived from the original on 28 May 2014. Retrieved 18 June 2014.