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Matt black glaze turned grey on red terracotta test tiles

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Quick question, its been bugging me all night. Took my test tiles out yesterday my black glaze has turned grey, why??

Technical information as follows:

Bisque fired red terracotta clay to cone 1 (1138 Celsius)

Glaze colour matt black (1020 -1100 Celsius)mixed from powder form 500grams to 608mls water, measured gravity 40 on hydrometer scale (light cream consistency)

Dipped test tile for 6secs

Slow glaze firing cone 03 to 1088  Celcius.

Did 6 different colours also same firing temperature as matt black(1020-1100 range) the pastel green has turned yellow orche , brownish when overlapped with white......  (p.s colours well well mixed for 2mins and mixed again when dipping each test tile for 6 seconds) 

Should I change the gravity to 50 or is the red clay overpowering the colours hence changing them???

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Six seconds seems long, but I'm not a Dipper.

I recently put some blue Mason stain in my local clay glaze and it goes Matt black, but there is a Matt ish sheen over it that, if fired hotter or cooler, or applied more thick or thin, I can see going "grey".

At this point I would test "amount" first, with a lesser dip time.



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4 hours ago, Bunbun888 said:

500grams to 608mls water

This sounds like to much water. I've never mixed a glaze with more water than dry glaze. Do you have an accurate scale? If so then measure the specific gravity of the glaze, generally they fall around 1.4 - 1.5 (this is a very loose ballpark range, have to fine tune it with your results). John Britt @ the 3 minute mark shows how to do this in video below. Also, after you have the glaze mixed up dip your test tiles with 1 dip over all, second dip covering 1/2 the test tile then dip a top corner of the tile a third time. This will give you info as to glaze thickness needed. Claybody makes a difference with glaze colour results. 

How readily does your cone 1 bisque soak up the glaze? Still fairly porous? Are the glazes sold as ^03 glaze? Just to confirm, these are sold as dipping glazes and not brushing glazes?

Welcome to the forum.


Edited by Min

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