G2934 Cone 6 MgO Matte Base G2926B Cone 6 Transparent Whiteware Base GA6-A/B Alberta Slip Cone 6 Base GR6-A Ravenscrag Cone 6 Base L3954B Mid Temperature Engobe Zero3 K Cone 04-02 Transparent Glaze G1947U Transparent Base - Cone 10R G2571A MgO Matte Base - Cone 10R GR10-A Ravenscrag Slip as-a-glaze - Cone 10R GA10-A Alberta Slip as-a-glaze - Cone 10R
GA6-A/B Alberta Slip Cone 6 Base
The base cone 6 glaze recipe, it is just 80% Alberta Slip and 20% frit (Ferro 3195 preferred, also 3134) and produces a well-melted amber transparent. It is very useful itself as a cover and liner glaze and can become much more if slow cooled (to form surface crystallization) and with additions (e.g. to form cobalt-free floating blue).
Transparent glaze of choice for dark bodies
The amber color and micro-bubble-free matrix make this work very well.
Clear glazes often do not work on dark bodies. The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body (M390) is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition.
Coffee Clay fired at cone 6 with transparent glazes. The Alberta Slip GA6-A glaze (left) is clearly better that the G2926B on the right. This firing is schedule C6DHSC, a 100F/hr slow cool to 1400F. The glaze is the right certainly has fewer bubbles than normal, but still is not nearly as good as the one on the left.
It is standard practice to fire cone 6 using a hold (or soak) at top temperature schedule (e.g. for 30 minutes), this reduces surface defects. However, to produce a defect-free glaze we recommend a drop-and-hold firing like the PLC6DS schedule.
The GA6-A glaze on the insides of these mugs has crystallized during cooling (it fast-cools as a transparent amber). This happened because we used the C6DHSC firing schedule, it drops at 150F/hr down to 1400F. The crystallizing in this recipe can be prevented by adding 1% tin oxide (the GA6-B recipe does not crystallize).
This glaze is most often prepared using the traditional method of simply adding water until the desired consistency is achieved (do the initial mix with 8 parts water and 10 powder). We find that 1.45 specific gravity works for us using our tap water. No flocculant additions are generally needed and application properties are very good as long as the slurry is not too viscous or too runny (dries quickly on bisque without cracking and, after dipping, there is minimal dripping).
Left: Raw Alberta Slip powder. Right: Roasted at 1080F. This is a plastic clay, thus it has a significant drying shrinkage. Where a glaze is applied thickly or the percentage of Alberta Slip is high, shrinkage cracks (which produce crawling during firing) will occur. We recommend a mix of roast and raw material in recipes. Roasting the Alberta Slip powder at 1000F greatly reduces the shrinkage. Use a firing rate and hold-time-at-1000F appropriate for the wall thickness and size of your bisque vessels (e.g. 500F/hr and 30 minutes for thin walled small vessels, slower and longer hold for large ones). If any of the powder within is black, increase hold time. Adjust proportions as needed (more roast if the glaze cracks on drying or more raw if it is drying too powdery or not bonding well).
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|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508