REPLICATING THE TEXTURES AND COLORS OF HISTORIC TERRA COTTA
Proper historic terra cotta replacement, restoration and repair requires an understanding of terra cotta colors and textures. An understanding of how they were produced and how they are replicated will help guide the restoration or replication process.
TerraGlas Terra Cotta Textures
Since architectural terra cotta was formed of clay, a variety of finishes and textures were possible. The texture is determined by the mold. Stromberg offers a range of replication options: GFRC, GFRP and GR Terra Cotta which all exactly replicate the textures of historic terra cotta. For highly detailed work Stromberg uses special silicone rubber molds to capture every minute detail of the original. So exact is the Stromberg process, that clients such as the Smithsonian, the US Park Service and even NASA rely on our mold and replication services. Molds are often made on site at the building being restored. In some cases, if there are no existing pieces of terra cotta to use for models, new models are sculpted in clay, based on old photographs and drawings.
Surface Finish of Terra Cotta
The texture of the architectural terra cotta is called the mechanical finish and is reproduced by the model and molds.
The surface texture of terra cotta is generally one of the following:
- Tooled or drove
- Eight lines to the inch
- Six lines to the inch
- Light irregular drag
- Heavy irregular drag, or combing
- Special-rugged, oak bark
The surface of most historical glazed terra cotta, whether lustrous or mat, is usually made smooth.
Colors and Finish
Historic architectural terra cotta finishes and colors can be classified into 4 categories:
- Unglazed terra cotta or terra cotta with a ceramic finish other than glazed, made in various shades of buff, gray, salmon, red, and brown.
- Glazed or enameled terra cotta, or terra cotta with an impervious ceramic finish of a glassy texture which may be either lustrous or mat, sometimes designated as full or dull glazes or enamels, made in various colors.
- Granite color terra cotta may be either:
- Unglazed granite color: A mottled ceramic finish similar to unpolished granite.
- Glazed or enameled granite colors: A mottled ceramic finish similar to polished granite, made either lustrous or mat.
- Polychrome terra cotta or faience, or terra cotta having two or more colors on the same piece, may be:
- Polychrome, unglazed, or unglazed terra cotta having two or more colors in the same piece.
- Polychrome, glazed, or glazed terra cotta having two or more colors on the same piece.
- Polychrome, blended colors, made only in glazed terra cotta. In polychrome glaze work where the colors are not separated by definite lines or contours or ornaments, but are to be blended together by brush treatment, the term "polychrome, blended colors" is used.
- Fire-gilded terra cotta. Fire gilding produced a coating of gold glaze, either mat or lustrous.
Coating or Spotting of Historic Terra Cotta
In terra cotta the original mixture of clay has little influence on the final color, which is obtained by tints applied in the form of slips, or glazes. These glazes produce the plain solid colors. The mottled terra cotta finishes are obtained by spotting the surface or the slip with various colors to produce effects such as the mottled colors of granite or other stones of a spotted nature. By combining spotting and texture, endless variations were possible. The replication castings by Stromberg use a similar process for reproducing the colors and glazes.
Age of the Terra Cotta
One issue that sometimes arises with terra cotta replacement and renovation is the question of age of the finish. All finishes collect some amount of dirt and grime over time and the patina of age can make new and old terra cotta elements appear slightly different. If the intent is to thoroughly clean the existing and remaining terra cotta (if any), then the exact same cleaning technique should be used on the terra cotta pieces that are used for the color match. In this way you help ensure that the colors of the new replacement terra cotta and the old terra cotta age together evenly. Stromberg goes to great length to coordinate its glazes with the existing terra cotta’s to provide for the most compatibility of color, both when the restoration is first done and for years to come.