What Are Crystalline Glazed Ceramics? How Crystals Form

If you have seen a pot or an urn covered in what look like frost flowers or lily pads under glass, you were probably looking at a crystalline glaze. This guide answers the question "what are crystalline glazed ceramics" in plain terms: what the glaze is, how the crystals grow, and why every piece is different. It then looks at the crystalline glazed urns for ashes we sell, who they suit and who they do not.

One note on the name. People sometimes search for "crystalline ceramics" and mean something broader. Here the word describes the glaze, the glassy coating on the outside, and not the clay underneath.

What a crystalline glaze is

Most glazes cool into a smooth, even layer of glass. A crystalline glaze is made so that part of it does something else: it separates out into crystals large enough to see. Digitalfire, a long-established technical reference for potters written by Tony Hansen, describes these as glazes in which large, multicolored crystals are grown on a very glossy glaze that is low in alumina and high in zinc, by controlling holds and soaks while the kiln cools (Digitalfire glossary, "Crystalline Glazes", read October 9, 2026).

The same page explains what the crystals are. In the most familiar crystalline glazes the main crystals are willemite, a zinc silicate. The glaze holds a high concentration of zinc oxide and enough silica for the two to join as willemite.

You can see the result on the Sodalite urn below. The round blue shapes are single crystals that have spread outward from a center point, and the darker areas between them are the glass they grew in.

Pictured: Sodalite Adult Cremation Urn for Ashes

How the crystals grow in the kiln

The crystals form after the hottest part of the firing, not during it. The glaze melts completely first. Then, as the kiln comes down in temperature, it passes through a band where willemite can begin and grow. Digitalfire gives 1900 to 2000°F as an example of this "zone of crystallization", which is about 1,040 to 1,090°C, and adds that some sources quote a wider range (same page, read October 9, 2026).

The potter holds the kiln inside that band so the crystals have time to spread. When they have grown enough, the kiln is cooled more quickly, which stops further growth and keeps the pattern as it is.

Two things make this harder than ordinary glazing. A crystalline glaze has to be very runny when molten, so it flows down the piece as it fires. It also contains little or no clay, and Digitalfire notes that this makes the wet glaze harder to keep mixed and slower to harden on the piece as it dries.

Why no two pieces look the same

A potter can choose the glaze recipe, the color and the firing schedule. What cannot be chosen is where each crystal starts or how far it spreads before it meets its neighbor. Those things happen in the kiln, out of sight.

Color adds another layer. Digitalfire notes that coloring materials tend to gather selectively, with one coloring oxide tinting the crystals and another the glassy areas around them. This is why a crystalline piece often shows two tones, one in the crystals and one in the background.

So a photograph of a crystalline glazed urn shows the design and the color family. The exact pattern on the piece you receive will be its own.

Crystalline glazed urns for ashes

A crystalline glazed urn is a ceramic urn finished in this kind of glaze. We have six at the time of writing: four for the full ashes of an adult and two small keepsakes. The sizes below come from each urn's own page.

Round urns with large crystals

Three of the adult urns share a rounded shape, a glaze with large, clearly separate crystals, and a lid in a dark finish that sits in the neck.

The Emerald Adult Cremation Urn is hand-thrown, with round green crystals over a blue-green ground. It holds 3.9 liters (about 240 cubic inches), stands 21 cm (8.27 in) high and weighs 2.2 kg (4.85 lb) empty. Its lid has a built-in tea light holder, so a candle can be lit on top of the urn.

Pictured: Emerald Adult Cremation Urn for Ashes

The Jadeite Adult Cremation Urn is also hand-thrown, in paler jade and sea-green tones. It holds 3.6 liters (about 220 cubic inches), stands 18 cm (7.09 in) high and weighs 2.4 kg (5.29 lb) empty. Like the Emerald, its lid holds a tea light.

The Sodalite Adult Cremation Urn, pictured earlier, is deep blue. It holds 4.1 liters (about 250 cubic inches), stands 22 cm (8.66 in) high and weighs 2.2 kg (4.85 lb) empty.

All three are filled through the top. Ceramic adhesive is supplied if you would like to seal the lid permanently, and the product pages suggest placing the ashes in a resealable bag and leaving the lid unglued if you would rather not. Our guide on how to seal a ceramic urn explains the steps.

The Violet Blue urn and keepsake

The Violet Blue Crystalline Glazed Cremation Urn looks different. It is a tall oval with a small lid, and its glaze is a fine speckle of violet and blue tones instead of large separate crystals. It holds 3.8 liters (about 231.9 cubic inches), stands 27 cm (10.63 in) high and weighs 3.6 kg (7.9 lb) empty. It is filled through the top and sealed with the supplied ceramic adhesive.

Pictured: Violet Blue Crystalline Glazed Cremation Urn for Ashes

The same glaze comes as the Violet Blue Crystalline Glazed Keepsake Urn, and there is an Emerald Green Crystalline Glazed Keepsake Urn in mottled green. Both are low, rounded pieces 6 cm (2.36 in) high and 9.5 cm (3.74 in) wide, with a space for a tea light in the top. Each holds 0.05 liters (about 3.05 cubic inches), which is a small portion of ashes and not the full amount. They are filled through a screw closure in the base.

Pictured: Emerald Green Crystalline Glazed Keepsake Urn for Ashes

Side by side

• Emerald Adult: 3.9 liters (about 240 cubic inches), top opening, lid with tea light holder, made for display in the home.
• Jadeite Adult: 3.6 liters (about 220 cubic inches), top opening, lid with tea light holder, made for display in the home.
• Sodalite Adult: 4.1 liters (about 250 cubic inches), top opening, made for display in the home.
• Violet Blue adult urn: 3.8 liters (about 231.9 cubic inches), top opening, listed for indoor or outdoor display.
• Violet Blue and Emerald Green keepsakes: 0.05 liters (about 3.05 cubic inches) each, base opening, tea light space, listed for indoor or outdoor display.

Who a crystalline glazed urn suits, and who it does not

These urns suit someone who wants the urn to be a piece of pottery in its own right, kept where it can be seen. They suit families who like the idea that the pattern exists once only. The candle designs suit anyone who would like to light a flame beside the ashes on a birthday or an anniversary.

They are the wrong choice in a few cases, and it is kinder to say so now.

• If you need an exact match to a photograph, or two identical urns, a crystalline glaze cannot give you that.
• If you plan a natural burial, glazed ceramic does not break down. Look at our biodegradable burial urns instead.
• If the urn will be carried a long way or handled often, remember that ceramic can chip or break if it is dropped, and the adult urns here weigh between 2.2 and 3.6 kg (4.85 and 7.9 lb) before they are filled.
• If you want to share the ashes among several people, each keepsake holds only a very small amount. Our guide to crystalline keepsake urns covers them in more detail.

Checking the size

The adult urns in this guide hold between 3.6 and 4.1 liters (about 220 to 250 cubic inches). Their product pages list them as suitable for a person who weighed up to about 99.8 kg (220 lb) for the Jadeite, 105.1 kg (231.9 lb) for the Violet Blue, 108 kg (240 lb) for the Emerald and 113.4 kg (250 lb) for the Sodalite. For a sense of scale, the Cremation Association of North America states that the average weight of adult cremated remains is between four and six pounds (CANA, "Cremation Process", read October 9, 2026). Urns are sized by volume, not weight, so use our urn size guide if you are unsure.

Looking after a crystalline glazed piece

A glazed ceramic urn needs very little care. Dust it with a soft, dry cloth and stand it on a level surface where it will not be knocked. If you use the tea light holder, do not leave a lit candle unattended.

You may one day notice very fine lines in the glass of a crystalline glaze. Potters call this crazing. Digitalfire notes that crystalline glazes have a high thermal expansion and tend to craze on most clay bodies, so it is a known trait of this kind of glaze.

If you are comparing ceramic finishes, our guide to saggar cremation urns describes another technique where the kiln, not the brush, decides the pattern.

Sizes and product details in this guide were checked against our product pages on October 9, 2026.

Frequently asked questions

What are crystalline glazed ceramics?

Crystalline glazed ceramics are pottery finished with a zinc-rich glaze that grows visible crystals on its surface as the kiln cools. The word describes the glaze, not the clay.

What are the crystals in a crystalline glaze made of?

In the most familiar crystalline glazes the main crystals are willemite, a zinc silicate. They form when zinc oxide and silica in the molten glaze join together during cooling.

What temperature do the crystals grow at?

The crystals grow while the kiln cools, not at its hottest point. Digitalfire gives 1900 to 2000°F (about 1,040 to 1,090°C) as an example of the range where they form, and notes that some sources quote a wider range.

Why is each crystalline glazed urn unique?

The potter controls the recipe and the firing, but not where each crystal starts or how far it spreads. The pattern forms by itself in the kiln, so it cannot be repeated exactly.

Can a crystalline glazed urn hold all of an adult's ashes?

The four adult urns in this guide hold between 3.6 and 4.1 liters (about 220 to 250 cubic inches) and are listed for the full ashes of an adult. The two keepsakes hold 0.05 liters (about 3.05 cubic inches), which is a small portion only.

Can a crystalline glazed urn be buried?

Glazed ceramic does not break down in the ground, so it is not suitable where a biodegradable urn is required. Cemeteries and burial grounds set their own rules about containers, so ask before you decide.