Comparing matte and glossy cone 6 glaze chemistry

Project Name

Compare the chemistry of cone 6 matte and glossy glazes

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UnAssigned

Notes

Notice the following differences in the chemistry:

The two mattes have much less SiO2 (the blue background number).

The G2934 matte has a lot of MgO, that is a matteness mechanism, producing a silky surface.

The G1214Z has a lot of CaO, that is a matteness mechanism because it encourages crystallization on cooling.

The mattes have less B2O3, the main melter. The glossy has much more, enough that the melt can take into solution much more Al2O3 and SiO2 than is common.

The mattes have a much lower Si:Al ratio, a classic mattness mechanism. If the MgO and CaO were not present it would have to be even lower to get an alumina matte surface.

The mattes have a much lower thermal expansion because they do not need a lot of KNaO, a major mechanism in producing gloss.

Cone 6 Magnesia Matte Base

Total:102.06

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Notes

*This matte recipe was developed at Plainsman Clays. It descends from a high-dolomite recipe that was originally used to compare shipments of dolomite for consistency. In our standard firings this recipe produces both good mattenss and a very functional surface coupled with very low chance of crazing. It is not as interesting as reactive mattes but this is better for functional and durable surfaces. This is also adjustable, the degree of matteness can be controlled by blending in a glossy.

WARNING: The degree-of-matteness is very dependent on cooling rate in the kiln. Fast cooling (e.g. our PLC6DS firing schedule in a lightly-loaded or smaller kiln) produces a silky matte or even glossy surface. Slower cooling (e.g. a heavily loaded kiln or the C6DHSC schedule) may produce a matter surface than you need (which is more subject to cutlery marking). Control the degree of mattness by either adapt the firing curve or blending in some glossy G2926B (simply the slurries pouring together, volumetrically is a good way to determine the ratio needed).

https://plainsmanclays.com/g2934 documents this recipe using a variety of stains.

Pictures

G2934 with Hemlock Green, Pansy Purle Stains

10 gram balls have been melt down onto a tile at cone 6.
Top: G2934 with normal flow
Left: 8% 6213 Hemlock Green. Needs significant flux.
Right: 8% 6305 Violet stain. Flowing a little less, needs a little flux.

True mattes should still be matte if overfired

The G2934 is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher (suggesting that its cone 6 matteness is due to incomplete melting). For the same reason the VC71, it is whiter in color (but as soon as it begins to melt and have depth the color darkens).

G2934 + 5% Titanium thinner/thicker on M390

Incredible cone 6 speckle body with G2934 matte glazes

This clay, L4115J3S, a Plainsman 3D-based body, fires vitreous and dense, yet there is no hint of bloating. With these matte glazes very durable and functional pieces are produced.

Outside glaze on both is G2934W (adds 10% zircopax). In our C6DHSC firings this produces as matte a surface as is possible without having excessive staining problems. To add a little gloss we blend in 15% of the G2926B Glossy clear.

Inside glazes:
Left mug: L4423A (85:15 mix of G2934 matte/G2926B clear glossy). It does not contain any zircopax.
Right mug: G2926B ball milled glossy, producing a striking visual yet highly functional surface.

These mugs look as close to cone 10R dolomite-glazed ware as we have ever seen! Especially the L4423A recipe

Melt flow comparison: G2934 with Frit 3124, Frit F-19

Fusion Frit F-19 is giving a more fluid melt.

The difference cooling-rate makes

These are the G2934 black glaze at cone 6. The piece on the left was fired using the C6DHSC firing schedule (drop-and-hold at 2100F then 150F/hr to 1400F). The one on the right was fired using the PLC6DS schedule (drop-and-hold at 2100F then free-fall from there). The slow cool gives the glaze on the left time to crystallize, creating a stony matte.

G2934 Cone 6 Matte + 4% iron oxide

Left: PLC6DS firing
Right: C6DHSC firing

Cone 6 Calcium Matte Base Glaze

Total:105.00

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Notes

*Because this was shrinking too much on drying (causing cracking), this recipe was changed in Mar 2019 (using code number G1214Z1) from 37 EPK to 20 EPK and 13 calcined kaolin (although 20+13 does not equal the original 37, it is correct because the calcined version loses less weight on firing). If unavailable, you can make your own calcined kaolin by roasting the powder in a container in a bisque firing.

This is a calcium matte (as opposed to magnesia matte). As such, it develops its visual effect by the crystallization of calcium silicates (which depends on the melt being quite fluid, or runny). A fortunate side effect of this mechanism is that additions of colorants and opacifiers can produce very interesting visuals that vary with thickness and firing cooling rate (which gets potters excited but which industry avoids because of the difficulty in maintaining consistency). There is an implication of this: Uneven thickness with this produces variations in the fired product (e.g. runs are very visible).

The spirit of this recipe originally was to demonstrate the main difference in the chemistry between a matte and glossy glaze, the SiO2:Al2O3 ratio (and how it can be changed to adjust the degree of matteness of a glaze). It was the product of a training course adjusting the chemistry of a glossy as a use-case for early versions of Digitalfire Insight chemistry software during the early 1980s). That recipe jumped from computer screens to actual use and has been employed around the world since by artist potters and even industry (especially tile).

Higher levels of MgO (as opposed to the CaO in this one) produce the other main type of true matte glaze (also well-melted yet matte), the surface of these is "wavy" that imparts a silky feel (as opposed to this calcium matte which has a smooth-feeling surface). Consider testing the G2934 MgO-matte recipe beside this one to determine which is best in your application. As with all mattes, be sure to control the rate of cooling in production firings (and match it to that done during testing) to get the intended degree of matteness. Remember that you can adjust the chemistry to produce more or less mattness (by lowering and raising the SiO2:Al2O3 ratio). Or you can blend in a gloss glaze base.

Cutlery marking can be an issue with this type of glaze, especially for slower cooling kilns (where more crystallization occurs).

Because the high kaolin content can cause excessive drying shrinkage we adjusted this recipe in 2019 to code G1214Z1, that recipe employs a mix of raw and calcined kaolin to supply the oxides.

Pictures

Felt glaze variation with G1214Z on thick

3/8/2009 on ragingbowl's blogspot is getting this effect with this base?

G1214Z Cone 6 Base Matte Transparent Glaze vs 2928C

Despite being a matte this glaze is well melted.

3 opacifiers on Coffee Clay, M390 in G1214Z

G1214Z2 on M340, M390 at cone 6

Cone 6 Clear Glossy Base

Total:110.10

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Notes

*The base recipe of the product sold by Plainsman Clays.

There are lots of pages and pictures online about this recipe and stain-added versions (e.g. at digitalfire.com and plainsmanclays.com). The page at Plainsman Clays has mixing and usage information.

This recipe has proven to work very well as a general purpose cone 6 transparent. It works best if ball milled (for lowest possible thermal expansion, best surface quality, least surface defects, best clarity). Unlike many others, this has very high SiO2, that means the best possible durability. It also has significant Al2O3, further contributing to stability of the melt and glass.

Although EP Koalin is used here, we have produced even better results using Grolleg and New Zealand kaolins. Using all three it is possible to get a slurry with excellent suspension and applications properites if attention is paid to the thixotropy.

This fits most Plainsman Clays bodies (and those of other manufacturers). But it does not fit Plainsman P300, a cone 6 whiteware, unless ball milled or a 325mesh silica is used.

Some clay bodies having very low silica content may craze this, in this case consider using the G1214U recipe.

Pictures

URLs

Technical informat..at digitalfire.com

Mixing, adjustment..PlainsmanClays.com

Alternate Code Number:MG6CG

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Cone 6 Clear Glossy Base" id="180072" key="mzXNAHwE" date="2024-10-02" codenum="G2926B" picturebasename=""> <recipelines> <recipeline material="Nepheline Syenite" amount="18.300" tolerance=""/> <recipeline material="Ferro Frit 3134" amount="25.400" tolerance=""/> <recipeline material="EPK" amount="19.600" tolerance=""/> <recipeline material="Vansil W-30 Wollastonite" amount="6.900" tolerance=""/> <recipeline material="Silica 325 mesh" lookup="Silica" amount="37.600" tolerance=""/> <recipeline material="Silverline 303 Talc" amount="2.300" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2014-05-15, Modified: 2024-10-02 19:22:40