A method of decorating terra cotta clay tiles by flood-filling liquid glaze into areas delineated by colored wax or grease lines.
Using this technique, a heavy-line-drawing of ceramic-stained wax or grease, is screen-printed (or hand-applied using a slip-trailing bottle). Brightly colored glazes (which are water suspensions) are flood-filled (using the same type of slip-trailing bottle) into areas between the lines. The glaze runs up to the lines and stops (due to the natural repulsion between the water and oil), then dries quickly (due to the porosity of the tile). Firing the tiles burns away the carrier in the typically black lines, fusing them to the surface.
The history of this technique is Spanish and the Spanish language web has the best coverage on how to do it (being very open with details and touble-shooting of the process). Googling terms like "técnica cuerda seca" will find coverage, then use the translation facilities of your browser.
Challenges in this process are many.
-Glaze fit: Since the glaze is applied very thickly it is more important for it to have a thermal expansion compatible with the body (or it will craze or shiver).
-Oil carrier: Coverage in readily available references seems vague, likely because many different types of oil have been used. It seems logical that wax-based resists are going to repel the glaze the best. While olive oil certainly repels water-based glaze well, it is thin and in practice readily absorbs into the porous terra cotta losing its repulsion properties. Logically it would seem that the oil needs to be thick to give the lines body and keep them oily during glaze application (for maximum repulsion). But apparently that is not the case. Lined ware needs to be handled alot (to apply the colored glazes) so the oil needs to dry. And the product needs to be fluid enough to pass through a silk screen easily. Linseed oil has most recently been recommended to me as the carrier of choice. “Stand oil” is a thickened form of linseed oil and is apparently even better.
-Black line powder recipe: It is best to mix a black stain (rather than just pure traditional manganese dioxide, which has toxicity issues) with a balanced frit targeted at the temperature (e.g. Ferro 3195 for cone 04). Since black stains of various chemistries are available, it would be best to try them all to discover the best performer. At first it might seem that a clayless mix, having no hardening or dry adhesion properties, would not be an ideal melt-carrier. However other factors (e.g. repulsion to the glaze, easy mixing of the powder and oil) are much more important (the kiln will burn away the oil and the frit:color mix will stay in place just fine). Testing is required to determine the best mix of frit and stain. Too much frit and the lines will melt too much and bleed into the color zones during firing. Too little and the lines will not fuse enough to be durable. It would be most common for the stain percentage to be the majority of the recipe (e.g. 80 stain, 20 Frit).
-Screen printing: For this an oil carrier will be the best (wax will clog the screen). Developing a mix of color powder and oil that will screen-print easily can be difficult. Since the oil burns away you can focus on the printability of the mix without concern about the type of oil used.
-Line fusion: You cannot just use pure black stain powder, it will not melt and fuse to the body. You must develop a mix of stain and other materials that will be glaze-like so that it melts and fuses well enough to the body but does not melt so much that it bleeds into the colors or runs. It needs to be thermal expansion compatible with the body also. Typically it would be best to simply use a stiff matte base glaze that works well on the body and add as much black stain as needed (likely not more than 10%). It is better to use a black stain than a mix of raw metal oxide colors.
-Body: A terra cotta body is definitely the best, these fire much stronger at low temperatures than white-burning bodies. Often terra cottas can have impurities that gas during firing and can cause pinholing in thick glazes. Get the best quality body you can.
-Firing temperature: The higher you can fire the stronger the tile will be. Many terra cotta bodies approach stoneware strength by cone 02, but that strength is accompanied by greater fired shrinkage (thus greater chance of warping during firing). Cone 04 is most common for the brightest glaze and best strength. If you can make it work at cone 03 (pretty well any low fire glaze will be fine at cone 03) it will be even better.
-Evenness of glaze coverage: This is a matter of skill and learning to work deliberately and quickly. Get the best slip trailing tools available. Sieve glazes well and flocculate them enough so they are thixotropic and do not run too much.
-Making your own glazes: Commercial painting glazes are gummed so they will dry too slowly and the cuerda seca lines will not resist them well. It is better to mix your own with just water. Choose a fritted transparent recipe (not one with Gerstley Borate) that fits the clay body (does not craze) and add ceramic stains to that (if you do not have a recipe consider G1916Q, G3879 or Zero3 transparent). Choose a glaze that does not have too high a percentage of clay so it does not shrink and crack on drying (calcine any kaolin beyond 15%, any ball clay beyond 10%). Add zircopax for white (about 10-15%). Different stains require different percentages to get intense color (e.g. blue might only need 3% whereas yellow might need 15%). Use inclusion stains for bright colors (e.g. orange, red, yellow). Clear glazes cannot be chemistry-compatible with all stains (e.g. a pink might fire grey in a clear glaze lacking CaO or having ZnO), consult documentation from the stain company if needed. To opacify a color add zircopax (but not too much or it will lighten the color).
The mug is has an unglazed (bare porcelain) outside surface. The cuerda seca lines containing the glaze have a formulation to flux and melt more than what is typical in the classic technique on tiles. And this appears to be reduction-fired porcelain at high temperatures (because of the iron specks and grey body color and the metallic appearance of the lines), in contrast to the normal low fire terra cotta temperatures of typical cuerda seca. The formulation for the lines is likely very high in iron oxide (with some kaolin added for suspension, dry hardness, adhesion and paintability). The fill glazes are a transparent with added stain.
By Suhag Shirodkar of India.
FireClayTile cuerda seca process
Cuerda Seca tile information at wikipedia
Cuerda Seca page at BigCeramicStore
Cuerda Seca tilework pictures at Google