Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!

200 mesh | 325 mesh | 3D Design | 3D Printer | 3D Printing Clay | 3D Slicer | 3D-Printing | Abrasion Ceramics | Acidic Oxides | Agglomeration | AI in Ceramics | Alkali | Alkaline Earths | Amorphous | Apparent porosity | Artware | Ball milling | Bamboo Glaze | Base Glaze | Base-Coat Dipping Glaze | Basic Oxides | Batch Recipe | Bisque | Bit Image | Black Core | Bleeding of colors | Blender Mixing | Blunging | Body Bloating | Body glaze Interface | Body Warping | Bone China | Borate | Boron Blue | Boron Frit | Borosilicate | Breaking Glaze | Brick Making | Brushing Glaze | Calcination | Calculated Thermal Expansion | Candling | Carbon Burnout | Carbon trap glazes | CAS Numbers | Casting-Jiggering | Catch Glaze | Celadon Glaze | Ceramic | Ceramic Binder | Ceramic Decals | Ceramic Glaze | Ceramic Glaze Defects | Ceramic Ink | Ceramic Material | Ceramic Oxide | Ceramic Slip | Ceramic Stain | Ceramic Tile | Ceramics | Characterization | Chemical Analysis | Chromaticity | Clay | Clay body | Clay Body Porosity | Clay Stiffness | Clays for Ovens and Heaters | Co-efficient of Thermal Expansion | Code Numbering | Coil pottery | Colloid | Colorant | Commercial hobby brushing glazes | Cone 1 | Cone 5 | Cone 6 | Cone plaque | Copper Red | Cordierite Ceramics | Crackle glaze | Cristobalite | Cristobalite Inversion | | Crystalline glazes | Crystallization | Cuerda Seca | Cutlery Marking | Decomposition | Deflocculation | Deoxylidration | Differential thermal analysis | Digitalfire Foresight | Digitalfire Insight | Digitalfire Reference Library | Dimpled glaze | Dip Glazing | Dipping Glaze | Dishwasher Safe | Dolomite Matte | Drop-and-Soak Firing | Drying Crack | Drying Performance | Drying Shrinkage | Dunting | Dust Pressing | Earthenware | Efflorescence | Encapsulated Stain | Engobe | Eutectic | Fast Fire Glazes | Fat Glaze | Feldspar Glazes | Fining Agent | Firebrick | Fireclay | Fired Strength | Firing Schedule | Firing Shrinkage | Flameware | Flashing | Flocculation | Fluid Melt Glazes | Flux | Food Safe | Foot Ring | Forming Method | Formula Ratios | Formula Weight | Frit | Fritware | Functional | GHS Safety Data Sheets | Glass vs. Crystalline | Glass-Ceramic Glazes | Glaze Blisters | Glaze Bubbles | Glaze Chemistry | Glaze Compression | Glaze Crawling | Glaze Crazing | Glaze Durability | Glaze fit | Glaze Gelling | Glaze laydown | Glaze Layering | Glaze Mixing | Glaze Recipes | Glaze shivering | Glaze Shrinkage | Glaze thickness | Globally Harmonized Data Sheets | Glossy Glaze | Green Strength | Grog | Gunmetal glaze | High Temperature Glaze | Hot Pressing | Incised decoration | Industrial clay body | Ink Jet Printing | Inside-only Glazing | Insight-Live | Iron Red Glaze | Jasper Ware | Jiggering | Kaki | Kiln Controller | Kiln Firing | Kiln fumes | Kiln venting system | Kiln Wash | Kneading clay | Kovar Metal | Laminations | Leaching | Lead in Ceramic Glazes | Leather hard | Limit Formula | Limit Recipe | Liner Glaze | Liner glazing | Liquid Bright Colors | LOI | Low Temperature Glaze | Majolica | Marbling | Material Substitution | Matte Glaze | Maturity | Maximum Density | MDT | Mechanism | Medium Temperature Glaze | Melt Fluidity | Melting Temperature | Metal Oxides | Metallic Glazes | Micro Organisms | Microwave Safe | Mineral phase | Mineralogy | Mocha glazes | Mohs Hardness | Mole% | Monocottura | Mosaic Tile | Mottled | Mullite Crystals | Native Clay | Non Oxide Ceramics | Oil-spot glaze | Once fire glazing | Opacifier | Opacity | Ovenware | Overglaze | Oxidation Firing | Oxide Formula | Oxide Interaction | Oxide System | Particle orientation | Particle Size Distribution | Particle Sizes | PCE | Permeability | Phase Diagram | Phase Separation | Physical Testing | Pinholing | Plainsman Clays | Plaster Bat | Plaster table | Plasticine | Plasticity | Plucking | Porcelain | Porcelaineous Stoneware | Pour Glazing | Powder Processing | Precipitation | Primary Clay | Primitive Firing | Propane | Propeller Mixer | Pugmill | Pyroceramics | Pyrometric Cone | Quartz Inversion | Raku | Reactive Glazes | Reduction Firing | Reduction Speckle | Refiring Ceramics | Refractory | Refractory Ceramic Coatings | Representative Sample | Restaurant Ware | Rheology | Rutile Blue Glazes | Salt firing | Sanitary ware | Sculpture | Secondary Clay | Shino Glazes | Sieve | Sieve Shaker | Silica:Alumina Ratio | Silk screen printing | Sintering | Slaking | Slip Casting | Slip Trailing | Slipware | Slurry | Slurry Processing | Slurry Up | Soaking | Soluble colors | Soluble Salts | Specific gravity | Splitting | Spray Glazing | Stain Medium | Stoneware | Stull Chart | Sulfate Scum | Sulfates | Surface Area | Surface Tension | Suspension | Tapper Clay | Tenmoku | Terra Cotta | Terra Sigilatta | Test Kiln | Theoretical Material | Thermal Conductivity | Thermal shock | Thermocouple | Thixotropy | Throwing | Tony Hansen | Toxicity | Trafficking | Translucency | Transparent Glazes | Triaxial Glaze Blending | Ultimate Particles | Underglaze | Unity Formula | Upwork | Variegation | Viscosity | Vitreous | Vitrification | Volatiles | Water in Ceramics | Water Smoking | Water Solubility | Wedging | Whiteware | Wood Ash Glaze | Wood Firing | Zero3 | Zero4 | Zeta Potential

Crucible

In ceramics, potters make crucibles to melt frits, stains and other materials. Crucibles are made from refractory materials that are stable against the material being melted in them.

Key phrases linking here: crucible - Learn more

Details

Crucibles are most commonly used in metallurgy but are also important in ceramics. Potters and industrial manufacturers make crucibles to melt frits and calcine stains and other materials.

Crucibles must, of course, have a much higher melting point than the material being melted (or otherwise processed) inside of them. This resistance to temperature takes precedence over any other desirable property. The refractory character also imparts physical rigidity. Crucibles also must be resistant to corrosion of the material being processed (especially if it becomes highly fluid) so that they cannot dissolve their way through vessel walls. Be sure to do tests on smaller melts before attempting a new-material melt in a big crucible.

Another huge issue is thermal shock. Crucibles are often thick-walled and heavy, thus, by nature, they are susceptible to cracking if exposed to rapid temperature changes. So, by necessity care must be used when heating and cooling them. Ideally, crucibles should be made from a material having good resistance to thermal shock, but this is not always practical (e.g. alumina, zircon are extremely refractory but do not have good thermal shock resistance). In industry, crucibles are often held at temperature for their entire lifetime of use so as to avoid cracking during cooling and heating. Smelting crucibles are designed to avoid asymmetrical temperature gradients, and come with preheat treatment and filling recommendations.

You can easily make your own crucibles from materials that have a high melting point, prefiring them as far above the service temperature as possible. Simple fire clay is commonly employed. However, be sure the fireclay you use can withstand the temperature (just because something is called "a fireclay" does not necessarily mean it actually is). Fireclays are often high in quartz so thermal shock resistance will be poor. Fireclays may not fire as dense as needed, and thus will have some porosity. Fireclays can also contain iron particles and soluble salts that have the potential to contaminate certain melts.

Super duty crucibles can be made from pure alumina oxide or zircon, these are available at any ceramic supplier. They are non-plastic by nature, but when processed to sufficiently small particle size they can be surprisingly formable and castable (zircon opacifier is very fine, and so are common grades of alumina). Binders (e.g. bentonite, smectite) can also be added to make them plastic enough for forming (or even throwing on a potter's wheel).

Kaolin, ball clay and silica are the most commonly available ceramic materials and they are all very refractory (because of their purity). Just about any mix of them if more refractory than almost any fireclay. Actually, most fireclays are just coarsely ground ball clays. An equal-part mix of the three produces a body of great workability and is highly refractory. But, such a body does not have good thermal shock resistance. Porcelain is a common crucible material, it employs these three materials and adds enough feldspar to produce some vitrification. Other materials can be incorporated also (e.g. mullite, alumina, pyrophyllite, zirconium silicate). In all of these, the silica portion should be as fine as possible to aid in the formation of silicates (minimizing the amount of residual unreacted quartz particles).

As noted, a crucible being refractory has nothing to do with its thermal expansion. True, thermal shock resistance in a crucible is had in ware that is sintered, not vitrified or melted. And it is the thermal expansion characteristics of the individual grains that determine that of the whole vessel. For low expansion, the material of choice is tabular alumina. Its particle size distribution will determine the density and collective particle bonding. Binders are needed to form it but are kept to a minimum. Of course, the vast majority of companies and potters do not have kilns that can fire high enough to sinter-bond pieces, so a compromise is often in order: Fire-bonding the particles with a low expansion flux, perhaps small amounts of borax or a frit (e.g. Ferro 3249).

Cordierite is very refractory and has very good thermal shock properties and could be an ideal crucible solution for many types of melts. For others, it may not be as resistant to attack by the melt. However, the formation of cordierite crystals in the kiln is also beyond the temperatures achievable in most production or pottery kilns.

A good way to determine a crucible body recipe is to go to websites that sell crucibles for the purpose you need. They will have information in their descriptions that we tell you what they make them from.

Related Information

Making your own crucibles to make your own speckle

Tap picture for full size and resolution
Home made crucible

I mixed a cone 6 porcelain body and a cone 6 clear glaze 50:50 and added 10% Mason 6666 black stain. The material was plastic enough to slurry, dewater and wedge like a clay, dry and break into small pieces. I then melted them at cone 6 in a Zircopax crucible (I make these by mixing alumina or zircopax with 3-4% veegum and throwing them on the wheel). This material does not completely melt so it is easy to break the crucible away (and no zircon sticks). I then break the black up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. If specks fuse too much I can lower percentage of glaze (and vice versa). Of course, the particles are glass, jagged and sharp-edged so care is needed in handling them.

Links

Materials Calcined Alumina
Materials Zircon
Glossary Fireclay
In the ceramics industry, clays that are resistant to deforming and melting at high temperatures are called fireclays. Kiln bricks are often made from fireclay.
Glossary Cordierite Ceramics
Cordierite is a man-made refractory low thermal expansion crystalline solid that forms at very high temperatures (in the right mix of kaolin and talc).
URLs https://en.m.wikipedia.org/wiki/Hessian_crucible
Hessian Crucibles
By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk



https://digitalfire.com, All Rights Reserved
Privacy Policy