GP K-59 Pottery Plaster

Notes

Georgia-Pacific K-59 Pottery Plaster looks like the closest direct replacement for USG No. 1 Pottery Plaster. The published numbers are strikingly close:

Oxide	Analysis	Formula	Tolerance

Property	USG No. 1 Pottery Plaster	GP K-59 Pottery Plaster
Water : plaster	70 : 100	68–70 : 100
Vicat set	14–24 min	25–35 min
Dry density	69 lb/ft³	69–71 lb/ft³
Dry compressive strength	2400 psi	2000–2600 psi
Set expansion	0.21% max	0.22–0.27%

USG describes No. 1 as its standard high-strength pottery plaster for slip casting and jiggering. Georgia-Pacific's published K-59 data describes it as intended for all types of potteries and sanitaryware, producing smooth, pinhole-free casts. It can reportedly be mixed as low as 64 parts water where exceptionally strong, long-wearing molds are wanted, although the normal published consistency is 68–70.

The most important number may be the normal consistency. Both products operate around 70 parts water per 100 plaster. That strongly influences the resulting mold's porosity, absorption and strength. This means a potter accustomed to No. 1 should not have to fundamentally rethink mold-making technique when moving to K-59. The dry densities are essentially identical, and the compressive-strength ranges overlap almost perfectly. The expansion of K-59 is slightly higher, but not dramatically so. The obvious difference is set time. K-59's stated 25–35-minute Vicat set is noticeably slower than No. 1's 14–24 minutes. That may actually be convenient for larger molds, but production shops would need to account for it.

Related Information

2026 Great Plaster Squeeze - A Timeline:

The death of a key material in North American ceramics

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USG specialty plaster is entering a final-inventory phase in the ceramic supply chain, and rationing has begun at some suppliers even though the Southard plant does not close until November.

Aug 19: News from Australia

An Australian industrial supplier now states that USG is “phasing out its entire Industrial Products range” made at Southard, that USG has confirmed these products will not be manufactured at another site, and that USG is no longer accepting orders.

Aug 29: One supplier is imposing anti-hoarding restrictions

It will serve normal requirements of existing customers, but will not sell plaster to new customers or permit purchases substantially above usual volumes. It is investigating alternative sources. Another is now limiting Hydro-Stone purchases to 100 lb per customer per week. USG and CGC themselves still have active product pages for No. 1 Pottery Plaster, Hydro-Stone and Ultracal 30, with no visible discontinuation notice, so their public website continues to lag what distributors are reporting.

Aug 31: GP K-59 sold out at some suppliers

Several Canadian suppliers are already showing Georgia-Pacific K-59 as sold out. Another still has 10 of K-59 available, compared with 172 bags of USG No. 1 Pottery Plaster still in stock. That suggests demand may already be shifting toward K-59 faster than some distributors can replenish it.
A signal from Georgia-Pacific itself: Its Blue Rapids, Kansas Industrial Plasters facility, the plant historically associated with K-59, is actively hiring maintenance personnel, including for a rotating 12-hour schedule, with the job posting emphasizing equipment reliability, production optimization and operating performance. Georgia-Pacific’s official industrial-plaster site continues to present pottery plaster as a core business and says it supplies sanitaryware and dinnerware manufacturers and produces both alpha- and beta-gypsum plasters. I still find no announcement of a production increase, additional shifts, capacity expansion, new warehouses, or a specific commitment to replace USG’s Southard volume.

Sept 1: A distributor is out of USG products

It reports that it is completely out of USG No. 1 Pottery Plaster, Hydro-Stone and 20 Minute Casting Plaster, with only a limited amount of Hydrocal White remaining.

Sept 2: GP announces close of a plant

Georgia-Pacific officially announced that it will permanently close its Tacoma, Washington gypsum plant, affecting 125 employees. GP says weak repair/remodel demand is the reason, and importantly says it expects to continue satisfying customer demand from its remaining facilities. The Tacoma operation is a gypsum-board plant, not the Blue Rapids industrial-plaster operation associated with K-59, so this does not indicate that K-59 production is being cut. But it does show GP is actively rationalizing its gypsum manufacturing network rather than broadly expanding it at this moment.

Sept 6: USG online store has an “LIMITED INVENTORY” notice

It states that availability of USG plasters and gypsum cements is limited and that once a product sells out, it will no longer be available and will be discontinued. Interestingly, significant final inventory remains: 312 bags of No. 1 Pottery Plaster, 218 Hydro-Stone, 123 Ultracal 30 and 95 Hydrocal White, all in 47.5-lb bags. These are being offered for sale while supplies last. The main USG website and CGC pages still have ordinary-looking product listings for No. 1 Pottery Plaster, Cerami-Cal and other products, without conspicuous discontinuation notices.

Sept 9: Various distributors have no K-59 in stock

The persistent K-59 shortages at ceramic suppliers suggest that distribution or production is not yet keeping pace with the emerging ceramic-market shift.

Sept 14: Arcosa emerges as another replacement source

Brackers Good Earth Clays says it has opened an account with Arcosa Specialty Materials and placed its first truckload order for pottery plaster plus a product comparable to Hydrocal White, due to arrive in October

Sept 17: GP requiring full load orders, actively staffing

A distributor reports that GP is requiring no mixing of types on orders. GP is recruiting production at the Blue Rapids, Kansas facility specifically for continuous manufacturing of plaster products. 12-hour shifts, including weekends, holidays, and overtime.

Sept 21: Industrial supplier freemansupply.com posts USG closure notices

They state: "With the upcoming closure of U.S. Gypsum (USG), Freeman's plaster and cement supplier, we have been sourcing alternatives to continue supplying the industry with quality products. If you are interested in plaster or cement, please complete this form to be contacted when suitable alternatives are available".

Links

URLs	https://gpindustrialplasters.com/pottery-plaster/ Georgia-Pacific Industrial Plasters
Materials	USG No. 1 Pottery Plaster

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