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Tony Hansen's Thousand-Post TimeLine

I am the creator of Digitalfire Insight, the Digitalfire Reference Database and ... more


Lizella clay (right) vs. Redart (left) at four temperatures

Bars have been fired at cones 4, 2, 02, 04 (top to bottom). The Redart is much more vitreous and reaches almost zero porosity by cone 4 whereas the Lizella still has 11% porosity at cone 4.

Tuesday 24th November 2015

Measuring shrinkage/absorption test bars in 1983 at Plainsman Clays

Data for hundreds fired clay test bars was logged into a portable Epson Programmable HX-20 computer and uploaded to a Radio Shack TRS-80 Model III where is was stored first on cassette, then floppy disk, then a loop tape. That data was later migrated to the Digitalfire DOS 4Sight lab record keeping ... more

Thursday 12th November 2015

These two pieces will not mature to the same degree in a firing

Soak the firing 30 minutes to mature the mug and the planter will not mature. Soak 2 hours for the planter and the glaze may melt too much and the clay be too vitreous. This is a troublesome issue with electric kilns. Furthermore, they employ radiant heat. That means that sections of ware on the ... more

Tuesday 10th November 2015

A bentonitic clay that takes a long time to dry

I finally gave up trying to dry the inner section of this DFAC test. During that test the inner part of the disk is shielded from the air flow or heat lamp. This sets up a shrinkage gradient that encourages cracking of the sample. But with some clays drying can be so slow that it can take a days. Serious cracking and high drying shrinkage almost always accompanies this phenomenon.

Tuesday 10th November 2015

How do black, red and yellow iron additions compare in a glaze?

Example of 5% black iron oxide (left), red iron oxide (center) and yellow iron oxide (right) added to G1214W glaze, sieved to 100 mesh and fired to cone 8. The black is slightly darker, the yellow has no color? Do you know why?

Tuesday 10th November 2015

Plainsman A2 ball clay DFAC drying disk

This test shows the incredible dry shrinkage that a ball clay can have, indicating the stress it puts the clay under during drying.

Friday 6th November 2015

Firing shrinkage variation between various clays

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and diring shrinkages vary widely. Materials normally acting as fluxes (like dolomite, talc, calcium carbonate) are refractory here because they are fired in the absence of materials they react normally with.

Wednesday 21st October 2015

Can a decal firing melt a glaze? Yes!

While high temperature glazes will not soften in a 1500F decal firing many low temperature glazes will. And when they do they could bubble like the mug on the right has done (obviously the body below was generating gases, possibly steam because it may have been absorbing water in the months since it ... more

Tuesday 20th October 2015

How to interpret the crack in a DFAC drying disk

Drying disks used for the DFAC test are 12cm in diameter and 5mm thick (wet). A crack pattern develops in almost all common pottery clays as they shrink during drying. This happens because the center portion is covered and stays soft while the perimeter dries hard. This sets up a tug-of-war with the ... more

Friday 16th October 2015

Albany Slip DFAC dried disk

This shows the soluble salts in the material and the characteristic cracking pattern of a low plasticity clay. Notice the edges have peeled badly during cutting, this is characteristic of very low plasticity.

Friday 16th October 2015

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