|Monthly Tech-Tip |
Modified: 2018-06-16 22:45:56
Crystal clear industrial dinnerware glaze
|Fusion Frit F-524||17.80|
|Old Hickory #5 Ball Clay||7.70|
This is an industrial tableware glaze recommended by tech support at Fusion Frits. It not only fires hard and crystal clear but has outstanding suspension and application properties. You may think that you already have a good transparent cone 6 glaze, but try comparing it side-by-side with this (especially on a porcelain piece).
The magic for the way this glaze fires and its hardness and durability lie in the variety of fluxes in contains and the very low boron content coupled with high SiO2 and Al2O3. The fluxes most likely to create micro-bubbles at this temperature are sourced in a frit. It has the traditional CaO and KNaO, but the talc adds MgO and the frit adds SrO plus a tiny bit of BaO. This mixed-oxide effect produces a very well melting glaze yet having excellent body (considering it has only 18% of a low-boron frit).
Plainsmanclays.com makes this recipe as a premixed powder. But they will also have a stock of the frit if you want to make your own. The glazes section of thier site has additional guidelines on this use of this recipe.
Cautions In Mixing Your Own Glaze
-Screening at 80 mesh is required during preparation if your wollastonite has agglomerated (otherwise your slurry will be full of tiny lumps).
-Fusion recommends 325 mesh silica and A400 nepheline syenite, but we have been using regular 200 silica and 270 nepheline with good success for stoneware.
-The recipe originally used #1 Glaze Ball Clay, but we have switched to a very similar more commonly available product, Old Hickory #5 (it has the same kaolin-like nature of #1 Glaze). These two ball clays are dramatically better than others for suspending glazes, mix this with another and you may get a poor working-properties preview of this otherwise very good recipe. In addition, these ball clays are higher in Al2O3 than typical (29% vs 25%), so there is a slight chemistry impact in using another.
-Use your own bentonite if you cannot get the Milwhite Bentonite B. Milwhite is low in iron, use the cleanest bentonite you have.
To prepare it for use, target a specific gravity of 1.5 (divide the total weight of powder by 1.08 to derive the amount of water to use). Add a flocculant (epsom salts, vinegar) to make it creamy. See the thixotropy glossary entry link below for more information on doing this.
These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.
G2934 is a popular matte for cone 6 (far left). The mechanism of the matteness is high MgO content (it produces a more pleasant surface that cutlery marks and stains less than other mechanisms such as crystallization or insufficient melting). But what if it is too matte for you? This recipe requires accurate firings, did your kiln really go to cone 6? Proven by a firing cone? If it did, then we need plan B: Add some glossy to shine it up a bit. I fired these ten-gram GBMF test balls of glaze to cone 6 on porcelain tiles, they melted down into nice buttons that display the surface well. Top row proceeding right: 10%, 20%, 30%, 40% G2926B added (100% far right). Bottom: G2916F in the same proportions. The effects are similar but the top one produces a more pebbly surface.
Plainsman M340 Transparent liner with various stains added (cone 6). These bubbles were fired on a bed of alumina powder, so they flattened more freely according to melt flow. You can see which stains flux the glaze more by which bubbles have flattened more. The deep blue and browns have flowed the most, the manganese alumina pink the least. This knowledge could be applied when mixing these glazes, compensating the degree of melt of the host accordingly.
M340 Transparent Liner glaze fired at slightly lower than cone 6. Using these modest stain amounts the degree of melting of the glaze is not overly affected (these were balls, they flattened during firing). However as a glaze layer on a body, many of these will not be as dark as you see here.
These are Mason stains added to cone 6 G2916F clear liner base glaze. Notice that all of these stains develop the correct colors with this base (except for manganese alumina pink 6020). However caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.
We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company they need to know the chemistry of your glaze to help.
This sample of glaze was dried under a heat lamp to measure its water content. If a glaze that is this thick can crack this little during drying and adhere even to stainless steel there is absolutely no reason you need to suffer glazes cracking during drying on bisque ware. This one is very high in frit with about 15% No. 5 ball clay. Drying cracking problems can be fixed using Digitalfire Insight, it enables you to juggle a recipe to reduce and substitute plastic ingredients while maintaining the chemistry.
|Materials||Fusion Frit F-524|
G1216L - Transparent for Cone 6 Porcelains
Incorporates some MgO (at the expense of CaO, KNaO) to reduce the thermal expansion of G1214M 5x20 glaze.
L3341B - Alberta Slip Iron Crystal Cone 10R
By adding a little iron to 100% Alberta Slip you can make an iron crystal glaze.
G1216M - Cone 6 Ultraclear Glaze for Porcelains
Substitute for low expansion cone 6 G1215U, this sources MgO from talc instead of a frit
G1215U - Low Expansion Glossy Clear Cone 6
A recipe sourcing high MgO (from Ferro Frit 3249) to produce a low expansion glass resistant to crazing on lower silica porcelains.
How to Liner-Glaze a Mug
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
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