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Every time I put a crystalline glazed piece in the kiln to be fired, I can't wait to see what I'm going to get when I open the kiln.
It is like opening a present, you never know what it is going to look like.



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All About Pottery Glazes

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You can buy these glazes in any pottery or ceramic shop.
Crystalline glazes are the most fun to coat your pottery with because of all the different shapes that come out when it is fired.
Some of the large crystals grown in a glaze even look like flowers, and sometimes they often resemble lichens and three dimensional fans and feathers as shown in the picture below.
Lime Green Crystaline
Crystalline glazes are those in which the oxides in them melt and form into new and different patterns and as the glaze cools it looks like glass with crystals inside. A bunch of crystals too small to be seen individually will give you opacity and matt surfaces. Larger crystals can be grown upwards of six inches, appearing to float in a glassy background. The wonderful shapes, of distinct and different color from their background are the result of trial and error mixing of the glaze formula and the cooling rate of the kiln. They call it science and patience, we call it "look what we got with this mixture", and the beauty shows itself magically from the process. If you are experimenting on your own it is very important to keep notes on ingredients and amounts that you mix together and firing and pausing times on the kiln.

Crystal shapes are soft flat rounds which may collide with each other in clusters, changing their forms. Usually a central star or bundle of needles can be seen in a smooth area before a fibrous looking ring fans out like a halo. Further haloes edge the shape where it meets the background glaze.This shape is colored by the penetration of a coloring oxide into the crystal and is clearly seen against the background. There is often a delicate fringe of a slightly different color where crystal meets the background glaze. The haloes can be placed centrally or around the border of the crystal. Where the glaze is thicker, as in the well of a bowl, the three dimensional forms can be seen as fibrous fans filling the depth. Time is needed during the cooling of the glaze for crystals to form.

In the early stages of cooling, if the temperature is held around 1800 degrees F for about two hours, the crystals begin as simple needle shapes. These can fan out at each end into double axe head like shapes. These attractive and fascinating crystals can be frozen at this stage by cooling the kiln rapidly after this growing period. The fuller rounder shapes develop when the temperature is maintained for further crystal growing periods. These pauses are programmed into the cooling cycle of the kiln controller and may last from three to eight hours.

Porcelain is often the choice of body for use with crystalline glazes, because bright colors show up well and there is little contamination from the body during the slow cooling. The main glaze ingredient is a frit. This provides most of the glass which melts at the appropriate temperature.

Frit Formulas Commonly Used

Ferro Frit

Silica 69.8%
Sodium oxide 15.3%
Calcium oxide 6.3%
Aluminum oxide 3.7%
Boron oxide 2.6%
Potassium oxide 2.3%

Ferro Frit

Calcined zinc oxide 23%
Calcined china clay 3%
Flint 23%
Titanium oxide 4%

High Alkaline Frit


Silica 18%
Zinc oxide 24%
China clay 40%
Titanium oxide 8%

Oxides of copper, cobalt and manganese are added totaling a maximum of 8%.

Each glaze component has a particular role but these are not single elements and their contributions overlap. The frit is designed to make the glaze melt quickly at top temperature. This presents a fully molten mix which is immediately ready for the new bonding to be formed. The rapid firing up to and down from the top temperature stops the formation of a body glaze layer which stops the forming of large crystals. Zinc oxide combines with flint and provides the zinc silicate for large crystals. The china clay gives stability and hardness to the final glaze. The flint is almost pure silica. It can be a different type from that provided by the frit for the main ingredient and supplies the structure for the crystals. Titanium oxide contributes to the start of crystals. It also brightens colors and helps with the movement of color in the glaze.

The oxides or carbonates which lose their carbon and excess oxygen with the addition of copper, cobalt and manganese, color the glaze body or crystals and sometimes both in specific ways, according to their strengths.

Crystals grow in the cooling glaze by the isolation of particular oxides from the surrounding glaze. Zinc silicate is most often the material of large crystals. In the hot liquid glaze body, the glaze minerals are loosened from their original combinations producing a fluid mixture of individual parts. In a normal glaze, as cooling begins, these parts link together to form irregular chains. This creates the changing substance, glass. For crystals to develop, the temperature is held for those periods when the parts start into more specifically organized chains. They establish bonds, which produce lattice structures, which are the framework of crystals.

Blue Crystaline

This complex crystal above shows the fan-like growth and three dimensional appearance in the depth of glaze. The blue staining of the crystal is incomplete due to the small percentage of cobalt oxide in the recipe. The pot was glazed first with a Ferro Frit based glaze and over this, a High alkaline Frit base glaze. Each one contained 0.5% cobalt oxide and 3% manganese carbonate.

In order to color premature zinc silicate crystals, the coloring oxides must be able to fit into the lattice structure. To enter the crystal, the metal coloring substance must be able to occupy one of the six sites otherwise held by zinc in the zinc silicate lattice. Cobalt, nickel, copper, iron and manganese are transition metals and are adjacent to zinc in the periodic table. Their substance sizes are also similar to that of zinc. Therefore all these metals can enter and color the crystals.

Yellow Crystaline Tan Crystaline

The colors here are from ilmenite and cerium oxide which has properties similar to tin oxide.

Green Crystaline

This fringe edged crystal is approximately 1 1/2 inches across. It appears in a glaze with 1.5% vanadium oxide and 1.5% ilmenite in a Ferro Frit base glaze. This glaze shows how copper oxide can give a green stain to both crystal and glaze. The glaze has 3% copper oxide and 3% barium carbonate. The barium carbonate shifts the color towards turquoise.

Glazes are brushed on, thicker above than below to allow for considerable glaze movement. Calcining the zinc oxide removes water and helps to avoid flaking of the glaze. A binder is used to make the glaze less brittle. Every pot is fired on a catcher made to fit the piece. Surplus run off glaze is contained by the catcher which must be separated after firing. The glaze and foot are then ground smooth. This is a complicated process requiring specialized grinders for the particular shapes.

These glazes make your pottery kind of one of a kind because no two pieces will be the same. I like to give them for presents, and the people that receive them really enjoy them also. What I like best of all is opening the kiln to see what I got!! These glazes are the neatest and most fun of all.

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With the help of the internet, you can now purchase most if not all of your ceramic and pottery tools and supplies online. We are located far from any well supplied dealers and yet working with reliable ceramic and pottery suppliers online has allowed us to recieve most of our orders within a timely manner.

When you get all set up, just enjoy the hobby and have fun at it. Some people get pretty serious and start selling their creations at craft fairs and small stores, but others just like to create items for themselves, relatives, and friends. Whichever kind of handmade pottery you desire to endeavor, enjoy the hobby and have fun doing it.

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