The Secret to Perfect Terra Sig - A Ball Mill and a little Crocus Martis

I recently traded emails with Jim Gottuso, of Sofia's Dad's Pots fame, about how to make Terra Sig using a ball mill. His recent blog posts about Sig made me realize I should post my Sig information (Click here to see his post). My base is Pete Pinnell's well circulated recipe, which I copied below. In my first year of grad school I discovered ball milling as a way to increase the Sig's consistency, yield, and shine when burnishing. A ball mill uses a spinning drum to rotate milling media, usually steel or porcelain balls, to crush wet/dry solutions into finer particle sizes. If you don't have a ball mill you can build your own by following digital fire's instructions (click here) or buy one online.

The first step after getting the ingredients is measuring the capacity of your ball mill. Then divide the recipe so that it will fit in the ball mill. The duration of milling depends on the base clay you are using. Course clays with short particle size ranges (like red art) need to be ball milled longer than clays that naturally have a wide range of particle sizes (like OM4). Since Terra Sig is made from the finest particles of clay the end goal of ball milling is to increase the number of that size particle. Ball milling significantly increases yield but over milling can cause a host of problems (clay fit, won't burnish to a shine, etc). When clay particles are over milled they become an unusable mush. I attribute this to the particle loosing its hexagonal structure. You want the clay particle to become shorter in length but not loose its hexagonal shape.

This excerpt from F.H. Norton's Studio Potter Article explains clay particle size and shape.

The Clay Particle
The optical and electron microscope have made it possible to examine the clay particles of all sizes. It is found that each particle is a crystalline plate with an hexagonal outline as shown in Fig. 1. The average diameter of this plate is one micron [one millionth of a meter], a size so small it can only be observed by a high power microscope. Other particles in the clay are as large as fifty microns and as small as one tenth micron in diameter. The thickness of the clay particle is about one tenth the diameter with the faces flat and smooth.

In the clay mass most of the particles are stacked together somewhat as shown in Fig. 2. In most clays the clay material is kaolinite with the formula (OH)4 Al2 Si2 O5, but other clay minerals occur in small amounts.

When I make red art terra sigillata, I ball mill the clay, water and deflocculant for 12 hours before starting the separating process. I then take the mixture out of the ball mill and put it into a see through container. After six hours of settling I collect the liquid portion of the mixture. There will be a thick sludge at the bottom of the container that can be thrown out. If you desire only the finest clay particles, which will give a shinier burnished surface, then you can repeat the settling process for another six hours. Boiling off moisture, or adding water, after decanting are necessary at times to achieve the ideal specific gravity for burnishing - 1.15 to 1.18.

For a deep Maroon Sig I add one teaspoon of Crocus Martis per cup of liquid. Crocus Martis is a slightly soluble iron sulphate. (Click here for specs) I got color happy one day and added 3 teaspoons thinking it would be three times as good. Unfortunately over saturating the solution made the burnishing properties worse. This makes sense because I introduced a semi-course metallic particle into the solution. One teaspoon per cup is sufficient for any high iron/heavy metal colorant. When I apply the Sig to hard leather hard pots I burnish after each of the two layers. If you stick to this specific gravity range you can almost burnish to a glossy finish. It's remarkable how the Crocus Martis helps the shine. If you look at the mug above the dark brown color is the Sig. After burnishing it is water repellent on vertical surfaces and on the feet of Terra Cotta pots.

I'd love to hear other people's Sig experience. Post a comment if you have any tips to share.

Pete Pinnell’s Real Authentic Lip-Smackin’ Smooth Easy Terra Sigillata

color suggestions to 1 cup liquid sig:
white = + 1 tsp. Zircopax or tin.
off white = + 1 tsp. titanium diox.
green = + ½ tsp. chrome oxide
blue = + ½ tsp. cobalt carb.
black = + 1 tsp. black stain
purple = + 1 tsp. crocus martis

In a 5 gal. bucket put 28 lbs (28 pints or 3½ gal.) of water. Add 14 lbs. dry clay. XX sagger works well for white base, RedArt for red. Add enough sodium silicate to deflocculate (a few tablespoons). For red clays use 2 teaspoons sodium silicate and 1 tablespoon soda ash. Allow to settle. Overnight is average. Less plastic red clays (such as RedArt or fire clay may require only 6-8 hours, while very plastic clays
like XX Sagger or OM4 ball may take up to 48 hours). Remove top ½-b w/o disturbing the mixture (syphon off). This is the sig. . Throw the rest away; do not reclaim.

Terra sig is best when the specific gravity if about 1.15. Useful range is 1.1-1.2. Specific gravity is measured by weighting out 100 gms of water, marking the volume, and weighing the same volume of the sig. Divide the weight of the sig by 100. If too thin evaporate. If too thick allow to settle longer. Apply sig to bone dry greenware and buff. For “patinas” use 1 gerstley borate + 1 colorant as a thin wash over bisqued sigs, applied and rubbed off. Works well on textured areas.

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