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Monthly Tech-Tip from Tony Hansen

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Ferro Frit 3124 vs Fusion F-19 at cone 04

Melt flow test of Frit 3124 vs F-19

On paper, Fusion F-19 has a very similar chemistry to 3124. However, as can be seen here, it is flowing a little more and appears to have a lower surface tension. The glass is also more transparent and the entrained bubbles are bigger. The differences could be partly to Fusion using a different set of raw materials to source the chemistry or differences in their smelting process.

Context: Ferro Frit 3124, Fusion Frit F-19, Ferro Frit 3124 vs Fusion F621/19 at cone 04

Monday 17th May 2021

Fusion F-12 vs Ferro Frit 3134 at cone 04

Melt flow test of Frit 3134 vs F-12

On paper, Fusion F-12 has a very similar chemistry to 3134. And in practice it also appears very similar, although a little more melt fluid than the 3134.

Context: Ferro Frit 3134, Fusion Frit F-12

Monday 17th May 2021

Ferro Frit 3124 vs Fusion F621/19 at cone 04

Melt flow test of Frit 3124 vs F621/19

The Fusion one is flowing a little more. And it has larger bubbles, so Fusion must be using a different set of raw materials to source the chemistry (materials that either have higher LOI or decompose to produce gases later in the firing). They do not supply the chemistry of Frit F621/19 and it is not shown on their website in 2021. But they recommended it to us as substitute candidate for Ferro Frit 3124.

Context: Fusion Frit F621/19, Ferro Frit 3124, Ferro Frit 3124 vs Fusion F-19 at cone 04

Sunday 16th May 2021

Ferro Frit 3110 vs Fusion F-75 at cone 04

Melt flow test of Frit 3110 vs F-75

On paper, Fusion F-75 has a very similar chemistry to 3110. However, as can be seen here, it is flowing a little more and appears to have a lower surface tension. The bubbling character is also a little difference. The differences could be partly to Fusion using a different set of raw materials to source the chemistry or differences in their smelting process.

Context: Ferro Frit 3110, Fusion Frit F-75

Sunday 16th May 2021

Serious cracking in a crystalline-glazed P700 Grolleg porcelain. Why?

Three cracked crystal glazes porcelain vases

The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.

Context: How much feldspar should be used in a Grolleg porcelain?, Crystalline glazes, Body Cracking and Dunting During Firing

Sunday 16th May 2021

The glaze is flaking off this low temperature plate. Why?

The glaze on an organge plate is flaking off

At low temperatures glazes and slips/engobes are not stuck on nearly as well as with stoneware and porcelain. So the glaze fit has to be better (poor fit will be evidenced by flaking at the lip). But that is not what is happening here. In this case a pigmented slip, or underglaze, was applied first, at leather-hard stage (thus it is being used as an engobe). The integrity of two bonds must now be considered: Slip-with-body and glaze-with-slip. Slip-to-body bonding is never as good as glaze-to-body or glaze-to-slip. When an engobe, or underglaze, is refractory then the bond-with-body is especially poor. Ceramic stains are highly refractory in comparison with low-fire bodies, simply adding them to an underglaze base recipe will make it refractory also. In addition, stains vary widely in their refractory character and the percentage of stain needed varies greatly with color. Some underglaze manufacturers compensate by incorporating a compensatory percentage of frit in each underglaze recipe. Other manufacturers simply have one base and add all the colours to that. Claims that underglazes work well across wide temperature ranges do not get tested when they are brushed on as decoration, but when they are applied like this, as an engobe, disaster strikes! In this case we can see that the failure is occurring at the underglaze-body interface and the glaze/underglaze "sandwich" is releasing in large flakes.

Context: Underglaze

Friday 14th May 2021

By the magic of delflocculation, all this powder will mix into that water

Two almost-full containers of clay powder, one half full of water

Casting slips require a minimum of water. Amazingly, it is possible to get 3000g of M370 powder into 1100g of water! And the fluid slurry produced, 2250cc, still fits in the container. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to electrolytically repel each other! An awareness of “the magic” can help give you the determination to master deflocculation, the key enabler of the slip casting process. Determination? Yes, the process is fragile, must must develop the ability to “discover” the right amount of Darvan for your clay mix and water supply. And the ability to recognize what is wrong with a slurry that is not working (too much or little water, too much or little deflocculant).

Context: Deflocculants: A Detailed Overview, Understanding the Deflocculation Process in Slip Casting, Propeller Mixer, Deflocculation, Slip Casting

Sunday 9th May 2021

Measuring slip viscosity using a paint-measuring device

Measuring ceramic slip viscosity using a Ford cup

A Ford Cup being using to measure the viscosity of a casting clip. These are available at paint supply stores. This is a #4, it holds 100ml and drains water in 10 seconds (it has a 4.25mm opening). This casting slip has a specific gravity of 1.79 and we target a 40-second drain. That being said, if you are not working in a factory, if will be sufficiently to eye-ball the viscosity as you gain experience. If you are in a high-production situation, the seconds-value that this test produces gives you something to write down in testing records, producing an audit-trail for quality control and problem solving later. One thing to note: A slurry can gel while it is draining, if this happens the value produced is not valid. First adjust the rheology so it maintains viscosity throughout the drain time.

Context: Low Budget Testing of the Raw and Fired Properties of a Glaze, Understanding the Deflocculation Process in Slip Casting, Viscosity, Slip Casting, Specific gravity, Rheology, Deflocculation

Saturday 8th May 2021

Cuerda seca black-line recipe for low fire needs revision

Cuerda Seca test tile

All the YouTube videos I watched focussed on techniques of applying the lines and glazes. But what is the black-line recipe? Does it require gummed glazes? For this version 1.0 black-liner, I mixed enough wax emulsion to make it easy-to-apply using these needle-tipped applicators (80 mesh sieving was needed to make it flow well through the nozzles). At cone 04 the lines are bleeding excessively. This recipe is 50:50 Ferro Frit 3195 and Mason 6666 black stain (plus 2% Veegum to harden it and slow down the drying). Version 2 will need to reduce the frit or expense it with a filler, perhaps feldspar. Another issue was my colored fill-glazes, they have enough clay to suspend and harden them for normal use, however during application here the bisque pulls water out of them and they dry too fast. For future versions I will add VeeGum to slow the drying (I suspect CMC Gum additions would not resist well from the lines). Another issue: Even using pure wax emulsion, the lines are not resisting the glaze as well as I expected. Further testing will demonstrate if this can be improved by glazing sooner after line application (I waited a day for this test). Or will I need to use an oil base instead?

Context: Applicator tips and bottles for Cuerda Seca, Cuerda Seca

Thursday 29th April 2021

Are manganese speckled clay bodies a toxicity hazard?

A mug made of manganese speckled clay

Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness, opacity and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.

Context: Manganese Granular, G2934Y glaze on Standard #112 body at cone 6

Tuesday 27th April 2021

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