{"id":16287,"date":"2024-12-12T11:42:12","date_gmt":"2024-12-12T14:42:12","guid":{"rendered":"https:\/\/gustavoassisceramicas.com.br\/?p=16287"},"modified":"2024-12-14T22:27:23","modified_gmt":"2024-12-15T01:27:23","slug":"a-transformacao-da-argila-com-o-calor-o-que-acontece-com-a-caulinita-na-ceramica-artesanal","status":"publish","type":"post","link":"https:\/\/gustavodeassis.com\/en\/a-transformacao-da-argila-com-o-calor-o-que-acontece-com-a-caulinita-na-ceramica-artesanal\/","title":{"rendered":"How Clay Changes with Heat: What Happens to Kaolinite in Studio Ceramics?"},"content":{"rendered":"

In handmade ceramics, heating clay is not merely a technical process but a true metamorphosis. Kaolinite<\/b>, one of the most common components of ceramic bodies, undergoes a series of physical and chemical transformations when exposed to heat. These changes directly affect your piece's final properties, from strength to appearance.<\/p>\n

In this post, we will explore what happens to a typical clay, such as kaolinite, during heating, and how these processes influence handmade ceramics.<\/p>\n

The Structure of Kaolinite<\/b><\/b><\/p>\n

Kaolinite (Al2Si2O5(OH)4) is a clay with a crystal structure of TO layers<\/b>:<\/p>\n

\u2022Tetrahedral layer (T)<\/b>: With silica tetrahedra (SiO2).<\/p>\n

\u2022Octahedral layer (O)<\/b>: With aluminium octahedra (AlO6) coordinated to oxygen and hydroxyl groups (OH-).<\/p>\n

\u2022These layers are held together by relatively weak bonds, making kaolinite ideal for shaping and ceramic processing.<\/p>\n

\"\"<\/p>\n

Transformations during Heating<\/b><\/b><\/p>\n

As kaolinite is heated in the kiln, it passes through different stages of transformation:<\/p>\n

1. Loss of Free Moisture (~100\u00b0C)<\/b><\/b><\/p>\n

\u2022What happens?<\/b><\/b><\/p>\n

\u2022Free water in the clay's pores evaporates.<\/p>\n

\u2022Effect on the piece:<\/b><\/b><\/p>\n

\u2022The piece shrinks slightly but retains its structural integrity.<\/p>\n

\u2022Proper drying before firing is crucial to prevent cracking.<\/p>\n

2. Loss of Adsorbed Moisture (~300\u00b0C)<\/b><\/b><\/p>\n

\u2022What happens?<\/b><\/b><\/p>\n

\u2022Water molecules weakly bound to clay particle surfaces are removed.<\/p>\n

\u2022Effect on the piece:<\/b><\/b><\/p>\n

\u2022There is no significant change in microstructure, but the clay begins to harden.<\/p>\n

3. Dehydroxylation (~500\u2013800\u00b0C)<\/b><\/b><\/p>\n

\u2022What happens?<\/b><\/b><\/p>\n

\u2022Hydroxyl groups (OH-) in kaolinite's octahedral layers are released as water vapour:<\/p>\n

Al2Si2O5(OH)4\u00a0 –> Al2Si2O7 + 2H2O<\/p>\n

\u2022The kaolinite structure collapses, forming an amorphous phase called metakaolinite<\/b>.<\/p>\n

\u2022Effect on the piece:<\/b><\/b><\/p>\n

\u2022The clay loses plasticity and structural stability.<\/p>\n

\u2022A controlled heating rate is essential to avoid deformation.<\/p>\n

4. Formation of Aluminium Spinel (~900\u20131100\u00b0C)<\/b><\/b><\/p>\n

\u2022What happens?<\/b><\/b><\/p>\n

\u2022Metakaolinite begins to reorganise, forming the aluminium spinel<\/b> (Al2O3 , SiO2) phase, with a cubic crystal structure.<\/p>\n

\u2022Effect on the piece:<\/b><\/b><\/p>\n

\u2022The piece gains density and mechanical strength.<\/p>\n

\u2022Porosity begins to decrease, marking the onset of vitrification.<\/p>\n

5. Mullite Formation and Vitrification (>1200\u00b0C)<\/b><\/b><\/p>\n

\u2022What happens?<\/b><\/b><\/p>\n

\u2022The spinel reacts with remaining silica to form mullite<\/b> (3Al2O3, 2SiO2):<\/p>\n

Spinel + SiO2 \u2014> Mullite<\/p>\n

\u2022At the same time, some of the silica and feldspar in the body melt, creating a glassy phase.<\/p>\n

\u2022Effect on the piece:<\/b><\/b><\/p>\n

\u2022The piece becomes extremely strong, with low porosity and high density.<\/p>\n

\u2022Vitrification gives ceramics their characteristic gloss and durability.<\/p>\n

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Mullite and metakaolin crystals. Source: Homer, Jane and Frank.<\/p><\/div>\n

The Importance of These Transformations in Craft Practice<\/b><\/b><\/p>\n

1. Controlling Heating Rates<\/b><\/b><\/p>\n

\u2022Dehydroxylation (500\u2013800\u00b0C) and the volume changes associated with spinel formation require gradual heating to prevent cracks.<\/p>\n

\u2022A slow temperature rise is particularly important for thick or complex pieces.<\/p>\n

2. Effect on Shrinkage<\/b><\/b><\/p>\n

\u2022Most shrinkage occurs up to the spinel formation stage. Plan the initial size of the piece carefully to allow for this reduction.<\/p>\n

3. Final Properties<\/b><\/b><\/p>\n

\u2022Mullite formation at high temperatures is crucial to thermal and mechanical strength.<\/p>\n

\u2022If the firing temperature is too low, the piece may remain fragile because of insufficient vitrification.<\/p>\n

Summary: The Journey of Kaolinite<\/b><\/b><\/p>\n

Up to 300\u00b0C<\/b>: Loss of physically held water.<\/p>\n

500\u2013800 \u00b0C<\/b>: Dehydroxylation and formation of amorphous metakaolinite.<\/p>\n

900\u20131100 \u00b0C<\/b>: Reorganisation to form aluminium spinel.<\/p>\n

1200\u20131400 \u00b0C<\/b>: Mullite formation and vitrification.<\/p>\n

Each stage transforms clay from a fragile, plastic material into strong, functional and aesthetically pleasing ceramics. Understanding these stages allows you to adjust firing to create durable pieces with distinctive characteristics.<\/p>\n

Enjoyed the post? Share your experiences with firing in the comments!<\/p>\n

Reference:<\/strong><\/p>\n

    \n
  • Homer, Jane and Frank. The Potter\u00b4s Dictionary of Materials and Tecnniques.<\/li>\n<\/ul>\n<\/div><\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"","protected":false},"author":1,"featured_media":16317,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[212],"tags":[228,815,818,817,816,95,226],"class_list":["post-16287","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-quimica","tag-alumina","tag-caulinita","tag-cristal","tag-espinel","tag-mulita","tag-queima","tag-silica"],"_links":{"self":[{"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/posts\/16287","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/comments?post=16287"}],"version-history":[{"count":3,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/posts\/16287\/revisions"}],"predecessor-version":[{"id":16318,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/posts\/16287\/revisions\/16318"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/media\/16317"}],"wp:attachment":[{"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/media?parent=16287"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/categories?post=16287"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/gustavodeassis.com\/en\/wp-json\/wp\/v2\/tags?post=16287"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}