Crystalline Glazes
Crystalline is a (very fussy) type of ceramic glaze where crystals grow in the glaze during the kiln firing process. The glazes are formulated with raw materials, notably zinc oxide, in particular ratios that enable crystal growth potential . This potential is only realised when paired with a complex kiln firing schedule that has multiple temperature ramps to help the crystals to grow and mature into various shapes, sizes and colours. If either component — appropriate glaze chemistry and firing schedule — is absent, desired crystals will not form.
While there are a bunch of crystalline glaze recipes in books and on the web, many are essentially useless because they do not include the firing schedule. This is like publishing a cake recipe without the baking time and temperature. Much like baking a cake, there are indeed known temperature regions for certain chemical reactions. For example, most cakes bake at around 177-230C (350-450F) and we know it will never cook in a fridge no matter how long it is left in there. The same applies to firing glazes; there is information on what crystal formation temperature ranges to hit but within that there is a lot of room for creativity (and failure) to achieve crystals of desired sizes, shapes and colours. And unlike baking a cake, you can’t do the equivalent of opening the oven and sticking in a toothpick to see if the centre of the cake is raw and adjust accordingly. Necessary adjustments for crystalline are far from obvious and the kiln is ~1200C (~2300F) after all! All to say, crystalline recipes require a lot of experimentation and finesse to become even remotely successful in producing crystals at all.
I’m not sure why crystalline recipes are particularly secretive, but I would love to share the fruits of my labour. Below is a link to my Glazy* page where you can find the successful recipes from my crystalline journey as well as a porcelain recipe I developed to optimise for translucency and plasticity while balancing structural integrity to prevent warping. Please experiment with them and share widely with anyone who may be interested! Please also feel free to reach out; I am no expert but would be happy to share tips from my own experience and or receive advice :-)
*Glazy is a crowd-sourced open database of ceramic recipes with many wonderful tools that help with sharing knowledge, chemistry analysis, guiding recipe development and troubleshooting.
Resources:
Firing Schedules by Phil Hamling; recipes are not provided but still very insightful for visualising temperature ramps with crystal rings
Crystalline Glazes by Diane Creber
Stunning macro crystalline exemplars by Matt Horne
Unique matte crystalline exemplars by Ted Secombe
Crystalline jewellery by Jared Karnes
Crystalline course by Ceramics Materials Workshop (CMW); I have not personally taken this course but I have taken others with CMW that are all fantastic for anyone who wants to understand the chemistry behind their clays and glazes
For Flux Sake podcast also by CMW; ceramic chemistry, debunking myths and studio safety in general