USG No. 1 Pottery Plaster is gone. Here's our new plaster.

If you make molds, you've probably heard: US Gypsum (USG) has stopped making No. 1 Pottery Plaster. This summer USG closed its plant in Southard, Oklahoma, which made No. 1 along with Hydrocal, Hydro-Stone, Ultracal and its other specialty plasters. For decades No. 1 was the standard plaster for slip-casting molds in the US, so this affects nearly every pottery supplier, not just us.

Here's what we've done about it, how the new plaster compares, and what we suggest before you pour your next production mold.

Our new pottery plaster: Arcosa

We now carry Arcosa Pottery Plaster, made by Arcosa Specialty Materials in Oklahoma. Arcosa makes it for ceramic slip-casting shops. It is white, and you mix it at the same ratio as No. 1. Our first truckload is on its way from Oklahoma, in 1, 5, 10, 25 and 50 lb bags.

Will it be different?

That's the question we hear most. The honest answer: on paper it's very close, and only casting in it will tell you for sure. Here are the two makers' published numbers:

• Same mix. If you mix No. 1 at 70 parts water to 100 parts plaster, mix Arcosa the same way.
• Same strength, within normal variation.
• A little more expansion as it sets. You won't notice it in a bat or a one-piece mold. It can matter in a multi-part mold whose pieces have to register tightly.
• Absorption isn't on either data sheet. Absorption decides how fast a mold casts and how long it lasts, and neither maker publishes a number for it.

Arcosa's test of our first batch

Arcosa tests every lot before it ships. Here are the results for the lot in our first truckload (lot 101426, tested October 15, 2026; download the certificate):

• Set time: 23 minutes, hand mixed. USG listed No. 1 at 14–24 minutes when mixed by machine and noted that hand mixing takes longer, so expect a similar working time. Water temperature and how long you mix change set time more than the brand does.
• It needs a little less water than No. 1 to flow. At 70 parts water it mixes slightly thinner than No. 1 did. 70 is still within Arcosa's recommended 67–70. Less water makes a harder, denser mold that absorbs a little more slowly; more water does the reverse.

So we're testing it. In our own shop we're pouring identical molds from Arcosa and from our last USG No. 1, then comparing set, casting rate, release, surface and how the molds hold up over repeated casts. We'll post the results here.

How to switch

1. Do a test pour before you remake anything important. When it arrives, a 1 lb bag makes a test block. A 5 or 10 lb bag makes a small cup mold you can actually cast in.
2. Cast your own slip in the test mold and compare wall thickness at the same drain time, and how cleanly it releases, against one of your existing No. 1 molds.
3. Keep your mixing routine the same: weigh the water and the plaster, sift the plaster in, soak 2–4 minutes, then mix. Our plaster mixing instructions still apply.
4. Don't combine plasters within one mold set. Pour every piece of a multi-part mold from the same plaster so the pieces fit and absorb evenly. Molds you already own will keep working.

What about Hydrocal and Hydro-Stone?

Arcosa doesn't make a replacement we can buy for these USG gypsum cements. We have limited stock of both, and a purchase limit so more people can get some. When it's gone, we don't have a replacement yet. We're still looking, and we'll update this post when we find one.

A note while we're here: Hydrocal and Hydro-Stone are not substitutes for pottery plaster in slip-casting molds. They're much harder and absorb very little water, which is great for sculpture and master patterns but means a slip mold won't cast. For slip-casting molds, use pottery plaster.

Questions?

Call us at 888-915-1196 or stop by the store at 35 S Hanford St (Tuesday–Saturday, 9–6).