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Pieter Mostert

Member Since 09 Nov 2012
Offline Last Active May 24 2017 02:14 PM

Topics I've Started

Free And Open Source Glaze Software

16 May 2017 - 04:09 AM

I've been working on some software that helps you compose glaze recipes that have a specified oxide composition: given some set of ingredients, find which combinations of the ingredients give you the desired combination of oxides. Most of the recipes I come across use ingredients that aren't available here (I'm in South Africa), but I can reproduce them on the level of oxides. They don't always come out the same, but it's a good way of producing useable glazes.

Below is a screenshot of the user interface:


For each oxide and ingredient, the blue numbers on the left are lower and upper bounds that the user imposes, while the red numbers on the right are the calculated minima and maxima among all recipes that simultaneously satisfy each of the imposed bounds. The green polygon shows the possible values of the percentages of Potash Feldspar and Neph Sy (I still need to insert labels). In this example, I'm trying to reconstruct this recipe, but with Wollastonite instead of Whiting, and using my own potspar and Neph Sy instead of Custer, and a couple of locally available clays instead of OM4 Ball Clay. I've ignored MgO and TiO2, and slightly decreased CaO and KNaO, to allow for a bit of wiggle room.

If I increased the minimum K2O UMF value to 0.15, as in the recipe, the plot of Potspar vs Neph Sy diagram would change accordingly:



At the moment the program consists of a bunch of Python scripts, so most people will probably not want to instal and configure the extra bits and pieces necessary to run the program in its current state. I'd like to eventually turn this into a (free) stand-alone program you can run on your desktop computer, but since I'm not a professional programmer, I'll need some help to do this, and iron out some kinks along the way. If you're comfortable programming in Python, and have time to spare, I could use your help. You can find the repository on Github (published under the GPLv3), and details about the code structure and bugs in the README.pdf document. I've only tested it on my Windows 7 computer, and a couple of other people have had trouble getting it to work on their Macs, so be warned.

Cone 4 Oil-Spot Type Glaze

02 October 2016 - 04:02 AM

I seem to have stumbled on a type of oil-spot glaze.

8% RIO on white stoneware

I'm a bit hesitant to call it an oil spot, for a couple of reasons. Firstly, it doesn't come close to traditional Chinese oil-spot glazes; the spots are more of a dull grey than silver, although some of the larger spots have shiny black areas in the centre.

Secondly, it seems the mechanism that creates these spots is different from that of a traditional oil-spot, at least according to the most common explanation, which is that when Fe2O3 thermally decomposes to Fe3O4 at about 1230 degrees C, the bubbles of oxygen that form drag the iron to the surface, forming the spots. My glaze firing didn't go higher than 1130 C (but I went slightly over cone 4 by firing slowly). So what's forming the spots in my case?
A slightly different explanation of how oil-spots form is given by Joseph Grebanier, in his book on Chinese Stoneware Glazes.

1) Bubbles rising through the molten glaze reach the surface and burst there, leaving pits or craters in the surface at those points.

(2) As the firing continues, the more soluble and therefore more fluid, extra iron-rich portions of the surrounding glaze pour into these pits and fill them before larger-particled, less soluble portions of the glaze can move to them.

(3) As a result, when the firing is concluded at just the right stage, the pits are more or less filled with the extra iron-rich glaze material that has crystallized into patterns which are more reflective of light than the rest of the glaze.

(I don't have a copy of the book, but I found the quote in Carleen Devine's write-up on Oil-spots). I don't understand what Grebanier means by the more soluble portions of the glaze. It sounds as if the glaze separates into two parts. Could something similar be happening with my glaze?

What's really interesting is that there are places where the glaze has cleared up. The spots are still there, but you can see the clay-body through the surrounding glaze. The test tile below is the best example:

8% RIO on red earthenware
This has 8% added RIO on red eathenware. I don't like the blotchy effect this gives, but I'd love to know why it happens.


I should add that there are a number of cone 6 oil-spot glazes out there, and unless you're firing fast, you won't go over 1230 degrees in a cone 6 firing:
There's also this paper about an oil-spot fired to 1160C.

The recipe I'm using is:

Refined Overberg earthenware  50
Neph Sy                                       4.8
Wollastonite                                 5.7
Talc                                            16.2
FSB 510 Frit                              23.4
+ Red Iron Oxide                          0 - 14% in 2% increments   (best results obtained with 8% RIO)

Overberg earthenware is a local clay that fires to an orange-brown colour at cone 4. By refined, I mean removed the larger particles, like you do when making terra sig, except that my slip still contained some medium-size particles. Unfortunately I don't have an analysis for this clay, and I'm not even sure there is one. To get a rough idea of what the UMF might look like, I assumed Overberg earthenware was a combination of 3 other local clays, and got the following bounds for the glaze without the added RIO:
SiO2      2.9 - 3.1
Al2O3    0.40 - 0.53
B2O3     0.30 - 0.33
MgO      0.43 - 0.45
CaO      0.32 - 0.34
Na2O    0.10 - 0.10
K2O      0.11 - 0.15
Fe2O3  0.01 - 0.10
TiO2      0.01 - 0.02

SiO2:Al2O3  6.0 - 7.4


Any thoughts on what's causing the spots?