Making a Wedging Table, Mold, Plaster Mix - Overview

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Making Wedging table - Lakeside Pottery Ceramic school and Art Studio in Stamford Connecticut

A lot of people asked us about wedging tables and plaster molds / casts. This page will focus on building wedging tables but also will answer a lot of questions about making plaster molds and plaster surfaces for recycling clay. We use our wedging tables constantly 5-7 days a week and have to replace them approximately every four years.




The table where clay is kneaded must be sturdy and may benefit from being screwed to the wall or to the floor. Other design considerations include: Comfortable height (30 -32"), absorbent surface, cutting wire, and sufficient space to set scale. Some potters use the wedging table to dry recycled clay.

Wedging table wood frame: For our wedging or clay drying tables, we built a wooden frame (hard wood) 2" - 2 1/2" deep with a bottom made out of plywood. We use hard wood to avoid warping and insure that the screws that hold the frame together are not pulled out with the plaster tension stainless steel screws are preferred and will last longer (wood will be wet the majority of it's lifetime).

The plaster we used is "No.1 Potter's Plaster" that comes in 50 Lb. bags. A 50 Lb. bag fills up 1,150 cubic inches. For example; a frame 2' x 2' at 2" deep will add up to 1,152 cubic inches. If the depth is increased to 2 1/2", you'll need 1,440 cubic inches and therefore one 50 LB. bag will not be sufficient. A frame section should not be larger than 30" x 30" surface area. Larger section is more prone to cracking as illustrated in one of the pictures on right. Note that the bigger the wedging table, the more you have to be aware of how far you have to carry it when dry and ready for use - it gets really heavy!!

The proper plaster selection is essential. For wedging tables, we recommend using USG No. 1 Pottery Plaster (comes in 50lb Bags). #1 Pottery Plaster is highly uniform, noted for outstanding performance and long life. The standard of the industry - this is the best material available for sanitaryware and dinnerware casting. Used for making plaster bats, wedging boards, and working molds. This is a relatively soft plaster which may be used for carving.

Pottery Plaster Regular is 25% softer than No.1 Pottery Plaster and therefore will have shorter life if used as a wedging table but it is 12% better in absorbing water and will perform better if used to make surfaces for drying recycled clay.

Note: Wedging tables larger than 30" x 30" might crack with time and a divider is recommended (see pictures on right).



General Directions and Guidelines - Plaster Molding


Use only clean tap water or distilled water. Metallic salts, such as aluminum sulfate, can accelerate the setting time, and soluble salts can cause efflorescence on the mold surface. Variations in water temperature will affect setting time and can cause difficulty in the control of mixing time. The water you use should be at room temperature or 70°F (21°C). If the water is too warm, the plaster will set too fast and vice versa.

Use fresh plaster. Plaster is calcined, meaning chemically bound water has been driven off through heating. If the plaster has been sitting around in a damp environment, it will have lumps in it, in which case it is no longer usable. Pitch it. Use plaster that has been stored dry and is lump free.

Weigh out materials. Do not guess about the amounts of plaster and water you'll need. Once you start the mixing process, you do not want to go back and adjust quantities.

When using Pottery Plaster No. 1, mix 70 parts of water to 100 parts plaster. This equals 4.17 gallons water per 50 pounds dry pottery plaster. (When using Pottery Plaster Regular, the ration is 74 parts of water to 100 parts plaster).

To determine the amount of water you need, estimate the wedging table volume in cubic inches then divide by 276 to give gallons of water.

To determine the weight of Plaster No.1 needed, estimate the wedging table volume in cubic inches then divide by 23 to give plaster's weight in Lbs.

Add plaster to water. Slowly sift the plaster onto the surface of the water. Do not dump the plaster or toss it in by handfuls. Adding the plaster shouldn't take more than 3 minutes.



Soak the plaster. It is important to let the plaster slake. Slaking is the process of leaving the plaster in the bucket with the water to let the plaster itself eliminate air bubbles by soaking. I would usually wait 3-4 minutes depending on the water temperature. The soaking allows each plaster crystal to be completely surrounded by water and it removes air from the mix. Small batches require less soaking than large batches. If the soaking time is too short, it may contribute to pinholes; and if it is too long, it will contribute to fast set times, early stiffening and gritty surfaces.

Mix the plaster. Mixing the plaster slurry is the most important step in producing plaster molds or casts with maximum strength, absorption, hardness and other important properties. Changes in mixing procedures will have a great effect on the finished product. The basic objective is to uniformly and simultaneously wet each particle as best as practical.



When plaster is wet, a chemical reaction is occurring which produces heat. Protect your hands with gloves. Set-up time is affected by water-to-plaster ratio (less water = faster set-up but decreased strength). Temperature of water (warm = faster set-up), and speed and length of mixing (long/rough agitation = faster set-up). Set-up should be at room temperature without fans or drafts. To facilitate clean up, the mixing container may be treated with WD-40 or other release agents. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream.

Breakdown lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates longer mixing times produce tighter and less-absorptive molds.

Manual Mixing (not generally suitable for batches over 5 pounds) - using a spoon or wood trowel, mix from the bottom using a side to side motion. Circular motion will cause bubbles. Mix to the consistency of heavy cream.

Electric Mixer - Use an electric drill with a "Jiffy Mixer" or equivalent attachment. "Stir" at no more than 1750 RPM while holding the Jiffy Mixer at 15 degrees off vertical.

The mixing container should be solidly tapped or even dropped on the floor several times to help air bubbles rise to the top.

The plaster is ready to pour when mixing is complete. " Marking" is a way to determine when it is ready - when a finger is trailed over the surface of the plaster and a slight mark is left in the plaster.

Air bubbles Use Isopropyl alcohol (rubbing alcohol) in an old spray bottle. When the time is nearing completion for mixing shoot in a couple of spray bursts. This has the effect of breaking the surface tension of the bubbles at the surface and you'll see them disappear.

Pouring the mix. Pouring should be done in a comfortably quick, smooth, steady manner directed towards the center of the base of the wedging board frame. Shake the table with and pour into a corner of the wooden frame cavity that will not be threatened as much by bubbles that don't make it up in the shaking process. After the pour is made shaking the table a little more sharply and/or vigorously will help bubbles up. After the pour, another spritz of Isopropyl will help eliminate surface bubbles again.

- Batch size should allow for the pouring to be completed within 5 minutes after the slurry has been mixed.

- If strength is the most important factor, longer mixing times are suggested, taking care not to over mix into the setting action of the plaster, which decreases strength.



Clean up. It is much easier to clean wedging table frame, tools and floor from plaster spills and overflaw within the plaster's curing time (15-20 minutes). Do so with damp rags.

Drying time. Depends on air humidity, newly made wedging table should not be used for 2-3 weeks to let the plaster dry completely before use. Drying time may vary depending on environment's humidity.

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