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L3954B Mid Temperature Engobe

Description

Engobes (slips) are very widely used in industry (especially tile). They are opaque and can completely hide the body underneath. However engobes must be matched to the body and temperature they are used for (same firing shrinkage and thermal expansion). Their application and use are tricky, but learning is worth it. Engobes open up many design opportunities. We are not yet manufacturing this product but you can make it for yourself for now.

L3954A white engobe on M340 and M390. The engobe has been applied to the inside and wrapped over the rim to midway down the outside. The clear glaze on the inside is G2926B Whiteware transparent. The outside glaze on the right is that same clear with 4% added iron oxide. The outside glaze on the left mug is GA6A Alberta Slip clear using Ferro Frit 3195 as the flux.

Process Properties

Always stir the slurry just before dipping something (to ungel it). When the slurry is right (see mixing instructions below) it is only necessary to dip stiff, leather hard items for a couple of seconds. Hold items at an angle and shake and roll them enough to drain engobe so that it drips off at a single point. When the last drip falls turn the piece over and the slurry layer should gel and hold in place (with no runs).

Engobes have been applied inside (white) and out (black) on these M340 mugs (at the stiff leather-hard stage). The outside was done first by simply pushing the untrimmed mugs base-down into the black engobe (up to the rim) and setting them down on plaster (the engobe application re-softens them). After a couple of hours, they were attached to the wheelhead (lip down with water) and the base trimmed (exposing the M340 surface below). Then the white engobe was poured in, poured out and the rim dipped up to the black.

The M340 mugs have been engobed (with black) a then stiffened and the wheat design incised. They will dry lips-down without covering (being ready the next morning for bisque firing). The secret to the fast drying is waxing the handles at leather hard stage to slow down their drying (thus evening up the water content of the whole piece).

Recipe

For details on the recipe and how to use this please read the recipe page at digitalfire.com and this share at insight-live.com. The base recipe fires white however it is easy to convert it to black: Exchange the 10% Zircopax with 10% burnt or raw umber. Or, 5% black stain.

Mixing Instructions

It is vital that the slurry be correctly mixed to be able to apply it evenly and thickly enough. The specific gravity does not have to be held to an exact number as is common for glazes, but we find it best around 1.45-1.48. But, since the recipe has so much bentonite, it is impossible to mix it to this specific gravity (you would end up with too much water). Thus Darvan must be used to thin it during mixing, and more Darvan is needed than you might expect (as much as 20-30 grams per gallon). Mix and add Darvan until the slurry is more fluid than a glaze would be (stops motion about 5 seconds after stirring is stopped). In this consistency it should begin to gel quickly as soon as motion stops (this happens because it has so much bentonite). If you overshoot and get it too thin, see the next paragraph.

If the slurry gels too quickly after stirring, or goes on too thick, add a little Darvan (it can be very sensitive, perhaps only 1 or 2 drops for a gallon if the slurry is almost right). If the slurry goes on too thick or runs and drips too much, add a pinch of Epsom Salts while it is mixing under the mixer. This will gel it (but it could take 30 seconds to take effect). Again, be careful not to add too much or Darvan might be needed to thin it again.

We have a Youtube video on fine-tuning the thixotropy here

Gallery

These are all M390 clay fired to cone 6 (soaked 30 minutes at 2100F on the way down, then 100F/hr cool down to 1400F). Left: With L3954 white engobe inside, and midway down outside. The inside is glazed with G2926B Whiteware clear. The outside is the same glaze with 4% iron added (notice there are no bubble clouds with added iron). Center: Whole M390 mug glazed with G2926B clear (notice the bubble clouds). Right: White engobe applied as with the left mug, but the whole thing is glazed with GA6A Alberta Slip base glaze (but using Ferro Frit 3195 instead of 3134).

An incredibly deep and rich blue (with no cobalt). This is M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt Umber instead of the normal 10% Zircopax) was applied inside and partway down the outside (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue.

Left: L3954B white engobe inside and partway down the outside of a dark burning body (inside glaze is Plainsman whiteware clear G2926B and outside glaze is Alberta Slip Floating Blue). Right: M340 with L3954B black engobe (having 10% Zircopax instead of 10% burnt umber). The glaze inside and out is Alberta Slip base GA6A (using Frit 3195 as the flux).

Safety Data Sheet

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Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net