|Monthly Tech-Tip |
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 is pooled 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 pooled very thickly it is more important for it to have a thermal expansion compatible with the body (or it will craze or shiver).
-Carrier: It seems logical that wax-based resists are going to repel the glaze the best. But it appears oils are more popular. Coverage in readily-available references seems vague, likely because many different types of oil have been used. While olive oil certainly repels water-based glaze well, it is thin and in practice readily absorbs into the porous bisque 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 pooling (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. All of this being said, wax emulsion based mixes are working for us. They dry well, resist well.
-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 (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. 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 they will not fire-bond well to the body or fuse enough to be durable. For low temperature, start with a 50:50 mix. For stoneware a filler might be needed to prevent over-melting: e.g. 50:25:25 stain:frit:feldspar. Some clay will also be needed to create a good paste (e.g. 2-3% VeeGum, the more VeeGum the more water will be needed).
-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.
-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 or ware 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.
-Commercial painting glazes: These are gummed so they will dry slowly, that will be a benefit here. Of course they need to be thinned with water. Cuerda seca lines should resist them well.
-Making your own glazes: If you want to mix your own glazes you will need treat them as a brushing glaze (with added CMC gum). 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).
Cuerda seca is getting much more popular. These are for the lines and fill in. From Amazon. Various needle gauges are provided. It is clear to see why the resist slurry needs to be sieved and free of particles (they will clog the needles).
By Suhag Shirodkar of India.
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.
All the YouTube videos I watched focussed on techniques of applying the lines and glazes. But what is the black-line recipe? Does it require gummed glazes? For this version 1.0 black-liner, I mixed enough wax emulsion to make it easy-to-apply using these needle-tipped applicators (80 mesh sieving was needed to make it flow well through the nozzles). At cone 04 the lines are bleeding excessively. This recipe is 50:50 Ferro Frit 3195 and Mason 6666 black stain (plus 2% Veegum to harden it and slow down the drying). Version 2 will need to reduce the frit or expense it with a filler, perhaps feldspar. Another issue was my colored fill-glazes, they have enough clay to suspend and harden them for normal use, however during application here the bisque pulls water out of them and they dry too fast. For future versions I will add VeeGum to slow the drying (I suspect CMC Gum additions would not resist well from the lines). Another issue: Even using pure wax emulsion, the lines are not resisting the glaze as well as I expected. Further testing will demonstrate if this can be improved by glazing sooner after line application (I waited a day for this test). Or will I need to use an oil base instead?
FireClayTile cuerda seca process
Cuerda seca glaze application at Kibak Tile
The lines and glazes are being applied to already glazed and fired tile.
Cuerda Seca tilework pictures at Google
Cuerda Seca page at BigCeramicStore
Cuerda Seca tile information at wikipedia