Silica Phase Changes

Potters start off with low quartz as their raw material.

As the temperature rises to 573°C this changes instantaneously to high quartz, with a volume change of 4.8%. Because of this sudden and relatively large change in volume, significant stresses are introduced if there are temperature differences across the pot, as different parts will expand (or contract) at different times. As such, it is important to fire slowly when approaching and oing through this temperature, so temperatures can even out and stresses are minimised.

If the silica is pure, then the next transition is to high crystobalite, which occurs from 1050°C upwards. Although this results in a volume increase of 11.6%, conversion typically takes many hours, so there is time for the pot to adjust to the thermal stresses.

If there are impurities in the silica, then from 870°C high tridymite will form (the starting temperature changes according to the impurities present, e.g. 872°C for Na, 883°C for K and 1005. This has a volume increase of 12.4%, but again the change is very gradual, and so can be accommodated by the pot. As the temperature rises, the tridymite may change to crystobalite - this occurs at 1420°C in isolation, but in the environment typical of clays and glazes may start as low as 1100°C. There is a small reduction in volume with this transition, of less than 1%.

If any amorphous silica is present, e.g. from wood or plant ashes, these will generally form high crystobalite on heating.

If cooling is slow enough, the crystobalite and tridymite may have enough time to change back to quartz. However at the faster rate of temperature change found in a kiln they will change to the low forms. Crystobalite changes between high and low at 270°C, with a contraction of 2.8%. Tridymite is more complex, having at least 7 phases between the its formation temperature and roome temperature, with the first phase change occuring at 477°C. Although the overall contraction is similar to crytobalite at 4.2%, each transition has a volume change of well under 1%, so there is time for the pot to adjust.

On future heatings, and crystobalite and tridymite will change from low to high forms, so caution needs to be taken when approaching and going through their transition temperatures.

Because the transitions in a kiln firing are not symmetrical, when pieces are fired more than once the amounts of tridymite and crystobalite are likely to increase with each firing, so increasing caution must be taken to avoid breakages.

<HR>

Licence

Tim Thornton Ceramics Technical by Tim Thornton is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Permissions beyond the scope of this license may be available at https://tim-thornton.com/tikiwiki/tiki-index.php?page=Licence.
If you re-use any material from this web site, you must include an attribution statement as described in the licence, and state if the work has been altered.

You may release work that modifies or builds upon this work only under the same licence (or a compatable one, as defined in the licence).

This licence does not permit you to use this work for commercial purposes, i.e. for commercial advantage or monetary gain. If you wish to use the work in this way, please contact us to discuss it. You will need our prior permission and approval to do this.

Whilst we aim to ensure that all material is accurate at the time of publication, we do not accept responsibility for decisions based on information we make available. No content of this site may be taken to be legal or medical advice.

Please bear in mind that materials on the website are the most up to date versions, but items you have downloaded in the past may not be. The materials are updated from time to time. We recommend that you use the latest versions by referring to those published on our website.

Exclusions

Images – Images are not available for re use under the OGL because in most cases copyright is held by a third party. The Council does not grant anyone any rights to re use images.

Use of logos – The use of Tim Thornton’s logos on any document or in association with any information, signifies that the document or information has been prepared or approved by him. The logos are to be used only when reproducing materials unaltered from the web site, to show their original source. If you wish to use the logo in any other way, please contact us first to obtain permission.

Third party intellectual property/copyright – We do not have the legal authority to grant permission to re use documents where legal rights are owned by third parties.

Applications for re-use

If you wish to make a specific application to re use information which does not conform with the CC BY-NC-SA 4.0 licence terms, please contact us to ask permission and for details of what, if any, additional conditions and/or charges may apply. Your application should be in writing, specifying your name and address and, identifying the information source you want to re-use and the purpose for re-use.

Disclaimer

Whilst every care has been taken in the compilation of this website, and every attempt made to present up-to-date and accurate information, we cannot guarantee that inaccuracies will not occur. We will not be held responsible for any loss, damage or inconvenience caused as a result of any inaccuracy or error within these pages.

Links from our site may lead to pages maintained by other organisations. These are provided purely for your convenience, and do not imply that we endorse or support those organisations, the information on their pages, or their products or services in any way. Additionally, no responsibility is assumed by us for the contents of its pages.