Clear leaded glaze for cone 04

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This is about showing that glazes based on lead bisilicate frits can be safe and are unmatched in gloss, color hosting and transparency.

Lead Bisilicate Glaze

Code #


Materials Amt
*Frit B350 85.000
EPK 15.000



*This glaze is part of my efforts to demonstrate the a lead bisilicate frit can be used to make a transparent glaze impossible by any other means. And it can be safe.

Can we make a crystal clear for L215 or the L4170B body? This would make terra cotta so much more attractive as a medium for pottery.


Boron vs Lead glaze on l215

The mug on the left is a brush-on version of a G3879C, a boron-based clear glaze for cone 05. Three coats were applied and the often-encountered clouding occurred. The one on the right is an 85:15 lead bisilicate:kaolin mix. Three coats were also applied (by brushing also).

G3971 Lead Bisilicate Glaze on L215 Cone 04


G3971 Lead Bisilicate on various clays

Crazing on Buffstone
These are about 3 months old.

G3971B Lead Bisilicate Glaze passes lead check test


Slipware Pottery

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Lead Bisilicate Glaze" id="230431" key="4ATjzjCh" date="2024-03-14" codenum="G3971"> <recipelines> <recipeline material="Frit B350" amount="85.000" tolerance=""/> <recipeline material="EPK" amount="15.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2023-01-05, Modified: 2024-03-14 21:13:13

Cone 04 UltraClear Low-Expansion

Code #


Materials Amt
Fusion Frit F-524 750.000 73.17%
Fusion Frit F-69 140.000 13.66%
New Zealand or Grolleg kaolin (Grolleg Kaolin) 90.000 8.78%
Silica 45 micron (Silica) 45.000 4.39%


Auto Unity Formula

BaO 0.04
CaO 0.38
MgO 0.18
K2O 0.09
Na2O 0.10
(KNaO) 0.19
SrO 0.22
B2O3 0.74
Al2O3 0.47
SiO2 3.76


Si:Al: 8.0:1
SiB:Al: 9.6:1
R2O:RO: 0.2:0.8


6.2 (Molar:5.8)




0.00 per kg


*An adjusted version of G3879 to reduce thermal expansion. It switches the silica to 45 micron (325 mesh) for better dissolution in the melt. And it increases the frit low-expansion F-69 (the Fusion equivalent of Ferro 3249) at the expense of F-524.

This also switches to whiter burning kaolin (New Zealand), that improves transparency and removes the amber coloration.

It still fires to the same crystal clear on light burning bodies (but clouding on terra cottas unless applied thinly). And is fitting pieces better.

For a dipping glaze we mix 1500g powder to 1125g water (57%/43%) to get a specific gravity of 1.54. This produces 1.7 litres. This builds up thickness quickly on Snow clay (because it is so absorbent).

For a brushing version we mix 350g powder with 5g Veegum and 5g CMC gum, shake this an add it to 450 water and then mix in a kitchen blender. This produces a gelled slurry that paints well.


G3879C is firing much whiter, why?

Cone 04. All three clear glazes are on the same body. Left to right: Amaco LG10, G3879C recipe, Crysanthos SG213. The middle one employs Fusion Frit F-524, it is more expensive. But look at the benefit: It fires much more transparent so the piece is much whiter. And it is not crazing or pin holing. And it is glossier.

G3879C, G1916Q, G1916QL1 on L213 Cone 04

The move from ball clay to New Zealand kaolin (centre mug vs right mug) greatly improves the clarity (because of the reduction in iron and titanium).

G3879C vs. G1916QL on L4410K low fire white

The iron from the ball clay is the reason the glaze on the right is off-white.

Cone 04 G1916QL1 vs G3879C melt flow comparison

G1916QL1 is more melt fluid so it is likely useful before cone 04. But G3879C is also very fluid (it also has a higher surface tension). Both are producing textbook high quality flows.

G3879C, G1916QL1 on L4410L Bisque 1800 glaze cone 04

No crazing after weeks of use, ultragloss.
Frit 524 has much less iron, that is why that mug is so much whiter.
After several months at home the G3879C did craze.

G1916QL1. G3879C, G3879G1 on L213 tiles - Cone 04

These are all craze-free after several months.

G3879C on L4410P dolomite body at cone 04

The outside glaze is Spectrum 336. The G3879C is our recipe, a base transparent that we have been tuning to reduce the thermal expansion as much as possible to fit our 50:40:10 BallClay:Dolomite:Nepheline body. The result is glaze compression, it is now under enough compression on the inside of this piece that it is forcing it apart.

G3879C on L215, Buffstone at cone 02

Clouding badly on terra cotta, better on the white body.

G3879C on L4170B at cone 04 with L3685Z6 engobe

No crazing after six months or more.
An even but not too thick application was achieved by dipping, not brushing.


LDW - LOI/Density/Water Content


XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Cone 04 UltraClear Low-Expansion" id="202474" key="SMrqz9tZ" date="2022-12-19" codenum="G3879C"> <recipelines> <recipeline material="Fusion Frit F-524" amount="750.000" tolerance=""/> <recipeline material="Fusion Frit F-69" amount="140.000" tolerance=""/> <recipeline material="New Zealand or Grolleg kaolin" lookup="Grolleg Kaolin" amount="90.000" tolerance=""/> <recipeline material="Silica 45 micron" lookup="Silica" amount="45.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2019-04-15, Modified: 2022-12-19 22:22:22

Spectrum Clear 700 Dipping Glaze

Code #



*Spectrum mix #37779
Difficult to apply, it is sticky (like dipping ware in watered down carpenter glue), appears to contain a binder. Dries very slow.

Without creating some thixotropy using Epsom salts it is difficult to see how this would be useable. The specific gravity required so that it is not too watery would result in overly thick application unless bisque was very dense. It does respond to Epsom salts but not as well out our glazes.

If on thin the result as dazzling over underglaze colors and the 6318 Terrastone body at cone 03. It is very easy to get it on too thick. What appears to thin when glazing is not.

Cone 02 on 6318 TS is a little too high. Bright a clear but more tiny surface dimples.


Spectrum 700 on 6318 Terrastone at cone 03

It is on too thick on the mugs, goes cloudy.
The dish is thinner and looks very good. No crazing.

Spectrum 700 Clear (left) vs. G2931B (right).

The spectrum glaze is on too thick and is clouding. The Ulexite glaze looks very good beside it and has better application properites.

Cone 2 (left> vs cone 06 Spectrum 700 with underglazes

P6318 Terrastone body.
The chrome-tin underglaze is more refractory and is not making a body with the body (as can be seen by the color differences on the brush stroke on the right). Some glaze has flaked off.

700 Clear on P6318 cone 2 vs 06

The 06 is defect and bubble free. Cone 2 has significant micro pinholes.

Precipitate in Spectrum Clear 700

XML (to paste into Insight)

Born: 2016-01-13, Modified: 2016-08-23 13:15:26