Clay & Shrinkage

Grog Addition Calculator

Calculate how much grog to add to reduce clay body shrinkage to a target percentage.

Updated May 2026

Current Shrinkage

Target Shrinkage

Batch Weight

Grog Mesh Size

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Results

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Enter your measurements above and click Calculate.

What Is Grog?

Grog is pre-fired, crushed ceramic material that is added to a clay body to reduce shrinkage, improve thermal shock resistance, and provide texture. Because grog is already fired, it does not shrink during the firing process — adding it effectively "dilutes" the shrinkable clay matrix.

The Math Behind Grog Addition

The calculator uses a dilution model: the blended shrinkage is approximately equal to the clay shrinkage multiplied by the clay fraction in the mix. To achieve a target shrinkage:

grog_fraction = current_shrinkage / target_shrinkage − 1

grog_pct = grog_fraction / (1 + grog_fraction) × 100

Mesh Size Guide

Mesh	Particle Size	Best Use
Fine (60–80)	Very small	Throwing, tableware, surfaces that will be glazed
Medium (30–60)	General	All-purpose hand building, raku bodies
Coarse (10–30)	Visible texture	Sculpture, wood-fire, anagama, large pieces

lightbulb Quick Tips

check_circle Keep grog under 20% for wheel-thrown work — higher percentages make the clay uncomfortable to throw and can abrade your hands.
check_circle Use grog of the same or lower firing temperature as your base clay — mismatched firing ranges create weak spots.
check_circle For raku and wood-firing, coarse grog (10–30 mesh) maximizes thermal shock resistance for rapid cooling cycles.
check_circle Molochite (calcined kaolin grog) is white-firing and ideal for porcelain bodies where color matters.

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Frequently Asked Questions

What is grog and how does it reduce shrinkage? expand_more

Grog is pre-fired clay that has already undergone shrinkage. When added to raw clay, it does not shrink again on firing — it acts as a diluting agent that reduces the overall shrinkage of the clay body proportionally.

What mesh size grog should I use? expand_more

Fine grog (60–80 mesh) minimizes texture change and is good for throwing bodies. Medium (30–60 mesh) is general-purpose. Coarse (10–30 mesh) provides maximum thermal shock resistance for raku, wood-fire, and large sculptural work.

Does grog affect workability? expand_more

Yes — grog reduces plasticity because it is not plastic itself. Above 15–20%, throwing becomes noticeably harder. For hand-building and sculpture, up to 30–40% is workable. For production throwing, keep grog below 15%.