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Technical Tips Blog
Using Glaze Chemistry to replace Ferro Frit 3134 in three glaze types
Don't listen to people that say you can just replace frit 3134 with 3124 in glaze recipes. That is wrong. Frit 3124 has five times the amount of Al2O3 (the second most important oxide in glazes) and half the amount of boron (the main melter). The glaze chemistry approach is much better, and easier than you think. To be able to do it you need two other Ferro frits, 3110 and 3195. As it turns out, Frit 3195 is more important than is 3124! A key goal in the way it is done is to end up with at least 15% kaolin (to suspend the slurry). I have chosen three types of recipes to demonstrate, dealing with each requires a unique approach. Two of the calculations produce improved slurry properties and one a recipe of significantly lower cost. Stay tuned for a video on how to do this in your insight-live.com account. If you have a recipe that needs this, get an account, enter it there and I can help you do the calculation.
Context: Ferro Frit 3134, Ferro Frit 3195, Ferro Frit 3124, Is Ferro Frit 3124 a viable substitute for Frit 3134, Do you know the purpose of these common Ferro frits?, Substitute Ferro Frit 3134 For Another Frit
Thursday 25th February 2021
Wanna throw porcelain plates with thick bottoms and thin rims?
Then they may need a week to dry! This plate had a one-inch-thick base (while the rim is a quarter of that). During the first few hours a thin rim like this will dry quickly, leaving the base far behind. But as soon as it would support the weight of a cover-cloth I put it into a garbage bag and sealed and left it for several days. Even after that it did not detach easily from the plaster, even though the bat had been dry. When I did get it off the base was still quite soft but the rim was stiff enough to enable turning it over and trimming it (I endeavoured to create a cross section of even thickness). Then I dried it under layers of cloth for several more days. It took at least a week. Had I allowed the rim to dry out during the first few hours it would likely have cracked later on.
Wednesday 24th February 2021
G2926B transparent glaze, proven reliable and durable
While colorful glazes on the outsides of pieces get lots of praise and glory, the transparent or white providing the functional surface on the insides of pieces often gets little attention. Really, what good is an attractive piece if the food surface is crazed, pinholed, blistered, leached, cutlery marked. This liner glaze, G2926B, is special at cone 6. It is a good example of how I found a recipe, recognized its potential and tuned and adjusted it to be better. It has proven itself as a base to host all manner of colorants, opacifiers and variegators. One of the reasons it is so widely used is that it has a story, it is well documented, with a code number that Google indexes. Drinking from a mug having a quality functional surface instills pride as its maker. And it minimizes complaints from customers.
Monday 22nd February 2021
Milk as a glaze! How is that possible?
After watching a youtube video (link below) about a Karelian potter, who uses this technique to make cookware, I could not wait to try it. He unloads the ware from his kiln (which appears to be a standard electric top loader used by potters in the west), and while still hot he immerses pieces in a bucket of milk for a few seconds. When he withdraws them they are steaming. I mixed some 2% milk and cream (to get closer to the whole milk he was using) and cold-dipped an 1850F bisque-fired jar and tile (of Plainsman L210) for about a minute (to enable it to soak in as much as possible). The potter claims to fire his ware to 300-350 degrees. I fired 500F/hr to 612F (350C), then held for 10 minutes and shut off to free fall. And it worked beautifully, high enough to get lots of carbon (which is only on the surface), not high enough to burn it away. The surface is smooth and pleasant-to-touch, it is odor-free. The potter claims it retains this surface over many years despite repeated oven use. This clay body, L210, is well suited since it is very fine-grained and fires to such a smooth unglazed surface. And the carbon makes it much better. Indigenous cultures throughout history have learned how to prepare, cook and store food in terra cotta clays like this, they withstand thermal shock better than vitrified stonewares and porcelains. Of course, more testing is needed, I will report as I proceed.
Saturday 20th February 2021
A down side of high feldspar glazes: Crazing!
This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could likely tolerate a 20% addition of that quite easily. That would reduce running, improve fit and increase durability. If the crazing does not stop the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some chemistry in your insight-live.com account.
Saturday 20th February 2021
A glaze is showing unwanted streaking. Why?
This is a fluid melt cone 6 glaze with colorant added and partially opacified. It runs into contours during firing, thickening there (notice the darkening around the logo), this is a desired visual effect. However, notice that drips and runs coming down from the rim, they are producing darker streaks. This is an application issue. Glazes that fasten-in-place too slowly will drain unevenly on extraction from the bucket (after dipping). This can be solved by making a thixotropic slurry. If bisque ware is too dense, glazes have a more difficult time fixing-in-place in an even layer, especially if they have no thixotropy. If glazes lack clay (e.g. less than 15% kaolin) they do not gel as easily. Slurries containing too much gum dry slowly and drips are almost unavoidable. If the problem is too much melt fluidity, choose a more stable base glaze can really help. Just because melt fluidity is less does not mean that it will be less glossy.
Friday 19th February 2021
Six layers, 85% Alberta Slip in the glaze, yet no cracking. How?
Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. Yet this slurry is 85% plastic clay, it shrinks so much that it would be like a "dried up lake bed" on the first layer. By the second layer it would all just fall off! How was it possible to dip six layers here? A 1% CMC gum addition (via a gum solution). Gums are often added to low-clay-content glazes to dry-harden them. But with all the clay in this one, no help is needed for hardening. This is an incredible demonstration of the power of a gum as an adhesive and hardener: It has sufficient power to actually counteract drying shrinkage! Of course, there is a down side: A drying period is needed between each layer, the length depends on the porosity and wall thickness of the ware and the amount of gum. This also demonstrates the difference between the function of Veegum (and similar materials) with CMC. The former, if added to this recipe, would would gel the slurry, require more water and drastically increase the shrinkage, making the cracking even worse. Of course, one could simply use a mix of calcine:raw Alberta Slip to control drying shrinkage and gum would not be needed.
Friday 19th February 2021
How does one make an incredibly white engobe for low fire terra cotta?
I found six secrets. 1. A lot of Zircopax, in this case 20%. 2. The whitest burning materials. New Zealand Halloysite as the kaolin, Veegum as the plasticizer and Nepheline syenite as the feldspar. 3. A terra cotta body the engobe fits (has the same fired shrinkage). I found a mixture of these materials that fits the terra cotta bodies I am using at cone 04: L4170B (20% nepheline syenite, 55% New Zealand kaolin and 25% silica), L215 and L210. 4. 55% kaolin will certainly produce drying cracks, to stop it the secret is 3% Veegum (to gel the slurry) and 1% CMC gum (to glue it and slow drying). These make it paintable and work for dipping on bisque. It works so well that pour-glazing the insides of these two slip-cast pieces produced an absolutely even, yet very thin paper-white layer. This is an excellent base for clear-glazing and brushwork decorating. 5. A clear glaze and fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, preserving the color of the engobe. 6. The right firing curve: The 04DSDH drop-and-hold schedule.
Friday 19th February 2021
Cone 6 oil-spot glaze effect, what works and does not work?
Getting a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of both chemistry and logic. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB supergloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I decided that I wanted one with high surface tension (in the melted form). I created the G3912A by calculating in my account at insight-live.com, substituting as much CaO as possible for SrO. My firing schedule, PLC6DS, produced the tile on the far right. Jackpot! I tried that at cone 5, 6, 7 and 8 and it worked at all of them. Surprisingly, my other tests done later were failures. Why? I thought that the gassing of the strontium carbonate (as it decomposes and releases CO2) was a factor in the original success, but apparently the bubbles generated from the Alberta Slip, in the G3914A black, must be the key (the other two bases have a low LOI).
Monday 15th February 2021
A large glaze batch mixing error rescued using glaze chemistry
The person used Frit 3134 instead of 3124, it makes up 70% of the recipe. The glaze melted much more and ran off the ware. That sounds like an impossible-to-fix problem. It just so happens that these frits have very similar chemistry except for one thing: 3134 has almost no Al2O3. That means that kaolin can be added to the bad batch to source the missing Al2O3 (and replenish the shortage of SiO2 at the same time). Extra silica is also needed to restore the full SiO2. The new chemistry is not an exact match, the B2O3 is a little higher, but this should not be an issue. Of course a raw:calcine mix of kaolin is needed (to prevent the glaze from shrinking too much on drying and therefore cracking). From this calculation we can see that for every 100 grams of the original powder we need to add 10 EPK, 15 calcined kaolin and 17 silica. Of course, one would need to know the water content of the slurry, that is calculated as (weight wet - dry weight)/wet weight * 100. If the slurry was 50% water, for example, then every 200 grams of slurry would contain 100 grams of powder.
Monday 15th February 2021