Mark Gordon used a specialty mortar to attach bone-dry pieces together to form this sculpture.

One of the challenges of working with clay is timing, especially
when constructing a complex sculptural form. Making sure that all
components are at the appropriate moisture and stiffness level when
joining pieces is one of the key considerations that sculptors must
address. Sculptor Mark Gordon has adapted a technique he first observed
being practiced by traditional Egyptian brick makers for his ceramic
sculpture. The technique is a sure-fire and simple way to securely
attach bone-dry greenware to bone-dry greenware. It works great for
him, especially when he is under pressure to meet deadlines and
scheduling challenges. Mark shares his technique with us today. -
Jennifer Harnetty, editor.
Here’s a method of joining dry clay to dry clay for use in sculptural work. For simplicity’s sake, and to promote an even shrinkage rate and firing contraction, I use a slop-consistency mortar mix made from slaked scraps of the same clay body that the sculptural parts are made from for joining, bonding and surface coating. To create contrasts in color between the mortar and the clay, I often add a darkening oxide or colorant such as iron oxide, manganese dioxide or black Mason stain.

Mixing the Mortar Clay

To make the mortar for assembling dry greenware, begin by mixing slaked scrap clay with powdered clay and aggregate material. The sculpture and the scrap should be made from a stoneware body. The fire clay in stoneware helps to make the structure stronger, and using a high-temperature clay body extends the glazing possibilities later. The aggregate material is the key feature of this mortar. I use large quantities of non-plastic material such as mixed-sized sawdust or perlite, in a ratio of two parts aggregate to one part wet clay.

First, mix the clay slurry, adding water if needed until it reaches a consistency about midway between slip and plastic clay. Use a drill mixer or clay mixer for larger quantities, and a mortar trough (available at hardware stores) for smaller batches. If you plan to build large-scale forms, add a small handful of shredded nylon fiber to about 60 pounds of this wet mix to improve the clay body’s dry strength. Next, mix in the aggregate. Sawdust is my principal aggregate, as it is inexpensive and readily available. I add small quantities of perlite for its rough tooth, texture and toughness. Note: vermiculite (expanded mica used for gardening) should be avoided, because the clay may pit or flake after firing.

The final mortar mix is a stiff slop, similar to what I saw used by Moroccan artisan brick-makers. Though not as wet as slip, it is still far too wet to wedge. Due to the predominance of absorptive non-plastics, even slight aging causes the mortar clay to stiffen quickly. Despite this fact, you can leave the mixed clay out in the open air because the dried outer crust can easily be remixed or slaked in just moments. This clay is not fussy!

Joining
Joining pieces with this mortar works best when the parts are bone dry. The process works because the mortar has several specific qualities that help lock the parts together:

  • Rough clay provides many nooks and crannies that act as tiny, locking undercuts.
  • Nylon fibers in the mixture provide a Velcro-like effect.
  • The wet clay in the mortar causes the dry clay in the separate components to absorb water and slake around the edges.
  • Large amounts of aggregate minimize wet-to-dry shrinkage.



This article appeared in the September/October 2008 issue of Pottery Making Illustrated

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To work with the mortar, dip a piece into the mixture, then firmly press the piece onto the main structure. Smooth out the joins and compress the mortar slip into the seams. For a more integrated appearance, paint a layer of mortar slip on all component parts. After firing at any temperature, this mixture remains mechanically porous. The bonds between the sections rely on interlocking construction – a sort of “silico-organic” mortise and tenon, with the mortar locking into recesses within the forms.

Mark Gordon teaches ceramics at Barton College in Wilson, North Carolina. To see more of Mark’s work visit his website at www.markgordon.com. Photos by Ron Sowers, Steven Steward and Max Gordon.

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