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Minnesota Clay Co. USA Technical Info - Mixing Plaster

How do I mix plaster?
The easiest way to mix plaster is to place the water in a bucket and sift the plaster through fingers until it forms a mountain in the water with a "peak" rising above the water. Allow the plaster to sink. You should see a noticeable increase in the length of time the plaster floats on the surface before sinking as additions continue. When all the plaster is added and submerged, mix slowly but thoroughly, breaking up all chunks and try not to introduce air into the mix. Use your hand or a power mixer on a low setting. Continue mixing until the plaster thickens slightly or until a finger drawn across the surface leaves a trace trail, then pour slowly into your form. Tap or lightly shake the form to bring air bubbles to the surface. Let set, remove from form and dry completely before using.

How much water do I add?
The water/plaster ratio is referred to as consistency. It is expressed as parts water per 100 parts plaster.
Typical consistencies for Pottery Plaster (K60) range from 60-75 (60-75/100).
Slip casting molds water/plaster ratio of 1 lb. : 1-1/2 lbs.
Press molds, wedging surfaces and bats water/plaster ratio 1 : 1-3/8 lbs.
Typical consistencies for DensitePlaster (K5) range from 34-35 (34-35/100).

Which plaster should I use?
Pottery Plaster (K60) is primarily used for slip casting molds, press molds, wedging tables & bats. Developed as an intermediate grade plaster that provides improved absorption and longer mold life.

Densite Plaster (K-5) is an extremely hard, low porosity plaster for casting, sculpting and case molds. A base Densite gypsum cement, this practical product provides low consistency in a neutral material. It contains no accelerators, retarders or additives. Use Densite K-5 gypsum plaster cement in either straight-pour statuary or solid forms with predictable success.  Used as a basic ingredient in dental stones, investment plasters, etc. Blend with a molding plaster or other general-purpose plaster to increase strength.

Helpful hints:

  • Always add plaster to water

  • Avoid mixing air into plaster

  • Water temperature affects the "set up time of plaster" (warm=faster / cold=slower)

  • Use a clean container for mixing

  • Keep unused plaster in airtight container

  • Do not pour unused plaster down the drain

  • 1 pt. water = 1 lb. water

Mixing Plaster Instructions
Description Use Consistency
Vicat Set Time
% Set Expansion Dry Density
Expected Compressive Strength (PSI)
K-60 Pottery Plaster 67 25 - 35 0.18 70 - 72 2100 - 2300
Densite® K-5 Plaster 34 - 35 15 - 25 0.300 - 0.325 102 - 104 7000 - 8000