A slip is a suspension of very fine clay particles in water (Vandiver 1990, 109) and has the consistency of cream. Slip may and should be applied to an unfired clay vessel immediately after shaping. It will dry and shrink at approximately the same rate as the clay body, insuring a good fit and preventing the slip from cracking or flaking off. A slip used alone on earthenware is made more durable by firing. A slip used on stoneware is usually protected by a covering of clear glaze.
In pre-industrial times slips made from iron-bearing clays or white clays were used as paints for decorating earthenware. Another common use was as an overall coating for a vessel. A red slip over earthenware intensified the red color of the clay body. A white or light-colored slip disguised a dark-colored clay body and created an effective ground for painted decoration. White clay was often in short supply, and using it as a slip over the clay body rather than as the clay body itself was an effective way of stretching a precious resource.
Slips made from iron-bearing clays were used prominently by pottery decorators in the late phase of the Ban Chiang site in Northeast Thailand. Some earthenware vessels have slips painted directly onto the clay, while others combine red slip with white slip.
Some prehistoric or protohistoric traditions used red slip as an overall coating for a vessel. Potters at Ban Prasat and associated communities in southern Northeast Thailand burnished the still-moist red slip with smooth pebbles to create a glossy surface that presented a more intense red color when fired.
Historically, red slip was used on earthenware made in the Haripunchaya principality, which was centered in Lamphun in North Thailand. Earthenware continued to be a specialty of workshops in Lamphun after Haripunchaya was incorporated into Lan Na in 1292. Formerly known as "Late Haripunchai" ware, it used red slip coatings (sometimes burnished) as grounds for stamped designs inlaid with white or black slip (Di Crocco 2001).
Kilns at Myein Kein (Mong Kong) in Shan state, Burma, used burnished red slip on water bottles and jars that were sometimes fired black (see Blackening). The technique is also used at Mung Kung near Chiang Mai and in southern Yunnan province.
Red iron slip was used to decorate one type of earthenware cooking pot that was recovered from the river at Ayutthaya, Thailand, and is thought to have been made in the vicinity. The decorator used a turntable to paint even horizontal bands of red pigment above and below a row of stamped motifs on the shoulder. Red horizontal bands also appear as part of the decoration (likewise incorporating white slip and black iron slip) on spouted water bottles (kendi) sometimes identified (without confirmation) as having been made at Sukhothai. These vessels imitate the decoration executed on stoneware kendi made at the Sawankhalok kilns and, like them, date to the sixteenth century.
Brown iron slip was used in a distinctive manner to create the "chocolate base" or "brown wash" identified on wares made at northern Vietnamese kilns beginning in the thirteenth century. A dilute brown slip was brushed inside the footrim of the trimmed base while the inverted vessel was spinning on a turntable. The slip was not coated with glaze. Several reasons for this treatment have been proffered. A reasonable suggestion is that the "chocolate base" replicates the oxidized brownish appearance of the thin celadon glaze on the bases of Yaozhou wares (or Yaozhou-style wares made at kilns in Guangdong) known to have been imported into northern Vietnam (Dupoizat 1997; Morimoto 2007). The idea was already in place in the twelfth century at southern Chinese kilns, which used a coating of iron slip to approximate the coloration of the revered Yaozhou wares, and such bowls were imported to northern Vietnam (Dupoizat 1997, 107, 112-13).
The brown base took on a life of its own, however, continuing in use in the fifteenth century and appearing on northern Vietnamese ceramics decorated with cobalt (including the 1450 bottle in the Topkapi Saray Museum, Istanbul). The iron wash appears occasionally on wares made at the Sawankhalok and Kalong kilns (Pa Dong and Payoom groups) in imitation of Vietnamese models (Shaw 1989, 48). It also appears on celadon dishes made in the Irrawaddy Delta, Lower Burma-although there the model may be the oxidized base of Longquan celadon wares from the late Yuan and early Ming periods in the fourteenth and fifteenth centuries.
During excavations of stoneware kilns in Twante, Lower Burma, a small number of sherds were found that bore black iron slip as a ground to intensify the color of brown iron glaze (Myo and Rooney 2001).
Some design formats on earthenware pots of the prehistoric Ban Chiang tradition used white slip as a coating for the entire vessel and as a foil for decoration executed in red slip, or-together with red slip-as a paint for filling designs outlined with incising.
Although the presence of slip has not been identified in Angkorian ceramics, potters at kilns in northwestern Cambodia and southern Northeast Thailand made an analogously sparing use of scarce white clay by laminating it onto dark-bodied vessels to shape only the portions intended to be covered with green glaze. The portions made with dark clay were coated with iron glaze.
White slip was used on the earliest (so-called MON) dishes made at the Sawankhalok kilns in North-central Thailand in the thirteenth century. It was applied to the interior only, over a dark gray clay body, and sgraffito decoration was added by drawing through the wet slip with a pointed tool. On contemporaneous MON-Associated Stoneware (MASW), made with a lighter clay body, white slip underlay decoration applied with iron pigment. This same format was also used on the dark-bodied dishes made at the nearby Sukhothai kilns. The creamy slip was applied thickly by dipping the vessel. At MON kilns in Si Satchanalai , kilns in Lan Na (such as San Kamphaeng and Nan), and the Si Sattanak kilns in Vientiane, Laos, a coating of dilute slip was brushed on thinly to serve as a foil for pale green glaze (Hein and Thongsa 1989).
At kilns in North Vietnam during the Tran dynasty of the thirteenth and fourteenth centuries, white slip was applied to ewers and flat-based bowls as a ground for decorating with iron pigment. These vessels are also associated with the appearance of brown slip on "chocolate bases" (Stevenson 1997, 124). Later, white slip was applied over coarser-quality clay bodies to be decorated with cobalt.
White slip was used for slip-trailed decoration (applied through a tube in which the flow of the slip could be controlled) on jars of various sizes made in Lower Burma that were then coated with dark iron-brown glazes.
The Lamphun kilns in North Thailand sometimes applied two colors of slip to a vessel, then used a tool to swirl them together in feathered patterns.