The method of to beat gold and copper nuggets in order to make gold and copper sheets for covering Buddha images and Mahathat Chedi was alled 'sheet metal and repousse' decorated works'. It occured since Sukhothai and Lanna Periods. The evidence about this type of art work was stated in the inscription found at Wat Chang Lom, Sukhothai Province, dated in 1384, and in Jinakalmalipakarna, Pali-Buddhis literature, which metioned the ceremony of covering Phra Mahathat Chedi in Chiang Mai Province during the regin of Kin Tilokaraj (1442 - 1487).
Lacquering was applied first, and floowd by gold lef giding on metal sheets. The metal sheets were used for covering Buddhist monuments (stupa and chedi) and Buddha images. This method was called 'Bu Thong Changgo' or 'Bu Thong Sakko', which may have been literally corrupted from 'Suwan-Jaggo'. This word was stated in anancient book in Lanna Period, describing the changko sheet used for wrapping the Chedi at Phra That Lampang Luang Temmple, Lampang Province. There was another technique called 'Hum Phlang'. The metal lsheet was used to cover bronze of wood figures. Artisan would elaborately beat its inner surface until the pattern raised.
Sheet metal and repousse' decorated works were not only applied with stupa, chedi, prang and Buddha images, but also used with amyong (leave-like carvings along the roofs at the gable ends)., capital, royal utensils, such as Bussabok (a pavition throne), Royal Seat, Pralong Prakob PhraKoad (an inner layer of a royal mortuary urn), Than Phra Benja (a base of a royal mortuary urn), sword and sheath, Buddhist sealings, bowl with a supporting stand, tray silver and golden carps, etc. The most famous sheet metal and repousse' decorated work were the royal articles of use and golden utensils found inside the hidden chamber at Wat Ratchaburana, dated in Ayutthaya period (15th Century).
Nowadays, the concept of covering metal sheets on chedis was changed. Previously, only the top of the chedi was covered with gold sheet or Thang Chango. In contrast, Gold Sheet is used for wrapping the whole chedi from the pinnacle to the base. Some chedis that has never been covered with Thong Chagko before, are now restored, and covered with Chango Sheet.
Sheet metal and repousse' decorated works are conserved by the office of Traditional Arts (Chang Sib Mu). The office is in charge of creating valuble workpieces for HM King Bhumibol and the royal family on special occasions. Some of the are golden parasol (chat0, tray contained with silver and golden flowers, Phan Waen Fa (the golden tray), meatl lid and its based wrapping sacred books, golden Sema (Boundary markers) made in form of Dharmacakra (the wheel of the law), Noppharat Sirapon Mongkol (Royal head dressing decorated with nine type of aspicious gems), Phra Katha Chomphon (the royal scpter), the summer garment made of gold of Phra Buddha Phra Biddha Patima Suwansirichok Pho Po Ro.