Volcanic Ash (Pumice)
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You must test before you use it, at any scale. Raw materials, mined or man-made, are not consistent. Color, strength, particle size and chemistry can change from one bag to the next and from one supplier to another, even when the name on the bag stays the same. Before you use this material in a glaze, slip or clay body of any size, mix a small test batch, fire it on your own clay in your own kiln, and check the results.
VOLCANIC ASH - PUMICE:
Navajo Brand® Pumice is a near-white, volcanic glass that is widely used
in industrial and cosmetic products. Navajo Brand® Pumice is mined as
natural ore from open pits in Espanola, New Mexico. This ore is
transported to our plant in Ohkay Owingeh (formerly San Juan Pueblo)
where it is crushed, milled, dried, and processed through a modern
classification system, producing the rigorously controlled, consistently
uniform Navajo Brand® products. THE ONE OFFERED HERE IS FFF in chart below:
Frequently Asked Questions
No. Volcanic ash is ground natural glass from volcanoes. It's much more consistent than wood ash, though it still varies a little from lot to lot. Wood ash is what's left after burning wood: it's high in calcium, very alkaline and changes from batch to batch. The two make very different glazes.
It's a pumice grade. The more Fs, the finer the grind. FFF is one of the finer grades, fine enough for glazes.
Start by swapping 10 grams of the feldspar in a glaze you know for 10 grams of volcanic ash. Ash has more silica, so the glaze may come out a little stiffer or less glossy. Fire a test tile and adjust from there.
Feldspar is a ground rock that does three jobs at once. It helps a glaze melt (it's a flux), it brings silica, the main glass-former, and it brings alumina, which stiffens the melt so the glaze stays on the pot. That's why so many stoneware glazes are built on it, often 20 to 45% of the recipe. Each feldspar has its own mix of sodium and potassium, so swapping one for another changes how the glaze melts. Fire a test tile after any swap.
Glaze and clay recipes go by weight, not by volume, and they don't count water. Each ingredient is a share of the dry total. 2% means 2 grams for every 100 grams of dry ingredients. For a 500-gram batch, that's 10 grams. Colors and opacifiers are often listed as extras on top of a recipe that adds up to 100, written like "+2" or "add 2%." Weigh those the same way: 2 grams for every 100 grams of the base recipe. A digital scale that reads to 0.1 gram makes small amounts easy.
A test tile is a small piece of the clay you use, glazed and fired just like your real work. Raw materials vary from bag to bag, and results also change with your clay, how thick you apply the glaze and how your kiln fires. A test tile is the only way to know what you'll get before you glaze a whole batch of pots. Write what's on each tile on the back (an underglaze pencil works), and keep notes so you can repeat what works.
Yes, whenever you handle the dry powder. Fine dust is the main hazard with most raw materials, and some can harm your lungs over time. Wear a NIOSH-approved respirator (N95 or better) while you weigh and mix. A basic dust mask that isn't N95-rated won't protect you. Once a material is mixed with water, dust is much less of a problem, but wipe up spills with a wet sponge before they dry, and mop instead of sweeping. Don't eat or drink while you work, and wash your hands afterward. Check each product's Safety section for anything extra that material needs.
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