Tom Buck on fri 8 sep 06
Kathryn Hughes:
Perhaps you may benefit by obtaining a copy (via the WWW) of the
Periodic Table of The Elements. It would show that the elements of Group I
include the alkali metals: Lithium (Li), Sodium (Na), Potassium (K),
and others. And Group 2 lists the alkaline earth metals: Beryllium (Be)
(not used in claywork), Magnesium (Mg), Calcium, (Ca), Strontium (Sr),
Barium (Ba), and Radium (Ra) (not used).
Basically, the Table assembles similar elements in a Group, going
from low atomic weight, to high atomic weight. So, within modest limits,
an element low in weight can sub for an element higher in weight, or the
other way around, high for low.
Both Na & K are common is our daily lives: NaCl (common table
salt), KCl (potassium chloride which is used medically when a patient is
consuming too much NaCl and must lower its consumption). Sodium carbonate
(soda ash) is often encountered, and it produces a highly alkaline
solution in water, quite similar to lye (NaOH, sodium hydroxide).
Potassium behaves similarly. So it shouldn't be a surprise that Lithium
compounds will behave in a like fashion, although Lithium Carbonate (the
compound often used by potters) has low (but not zero) solubility when
used in a recipe and then water is added to make the glaze mix (a slurry).
If the slurry is left standing for some days/weeks, it could be come quite
alkaline and harm flesh. This doesn't happen with Na & K materials because
(other than soda ash or potash) we use feldspathic materials to source Na
& K in our recipes, and these materials have extremely low solubility.
In some recipes, the need to lower expansion/conraction of a
fired glaze requires that some Na &/or K be replaced with Li feldspar or
Lithium Carbonate. If the substition is modest, the recipe will yeild a
satisfactory fired glaze; however, if too much Li compound is used, then
shivering will occur (the glaze contracts too much).
as for the alkaline earths: Calcium compounds are the standard,
and give solid results in most cases. yet some special effects can be
achieved with Ba compounds (eg, high Ba will cause copper to make a blue
glaze, not the usual green effect). and glaze mixes that depart from
glossy surfaces, that is, a matte surface is sought, then the matte effect
can be achieved by using extra Ca, or extra Mg, or subbing either of
these, with Sr or Ba. The difference between Ba & Sr is that Sr++ ions can
be handled by the body with little ill-effects should the fired glaze be
affected by materials that cause Sr++ ions to form and migrate to the food
we eat. This is not the case for Ba++ ions; if ingested as such the ions
may result in the body being poisoned.
as a basic rule, always use Ca materials in glazes for Cone 6 or
lower when making food-ware. some lesser amount of Mg materials or Sr
materials can be used in Cone 6 mixes and mixes of Cone 10/+. and
generally most potters accept the use of Ba materials only when the glaze
will go on surfaces that will not come into contact with food.
til later. peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
Kathryn Hughes on sat 9 sep 06
Thank you so much, Tom, for the information! I have been out of the loop for quite awhile and am getting back into the swing of information about chemicals.My first thought after reading your message is "This man is a walking wikipedia!" LOL Again, thanks for the info. With warm regards, Kathryn in NC
Tom Buck wrote: Kathryn Hughes:
Perhaps you may benefit by obtaining a copy (via the WWW) of the
Periodic Table of The Elements. It would show that the elements of Group I
include the alkali metals: Lithium (Li), Sodium (Na), Potassium (K),
and others. And Group 2 lists the alkaline earth metals: Beryllium (Be)
(not used in claywork), Magnesium (Mg), Calcium, (Ca), Strontium (Sr),
Barium (Ba), and Radium (Ra) (not used).
Basically, the Table assembles similar elements in a Group, going
from low atomic weight, to high atomic weight. So, within modest limits,
an element low in weight can sub for an element higher in weight, or the
other way around, high for low.
Both Na & K are common is our daily lives: NaCl (common table
salt), KCl (potassium chloride which is used medically when a patient is
consuming too much NaCl and must lower its consumption). Sodium carbonate
(soda ash) is often encountered, and it produces a highly alkaline
solution in water, quite similar to lye (NaOH, sodium hydroxide).
Potassium behaves similarly. So it shouldn't be a surprise that Lithium
compounds will behave in a like fashion, although Lithium Carbonate (the
compound often used by potters) has low (but not zero) solubility when
used in a recipe and then water is added to make the glaze mix (a slurry).
If the slurry is left standing for some days/weeks, it could be come quite
alkaline and harm flesh. This doesn't happen with Na & K materials because
(other than soda ash or potash) we use feldspathic materials to source Na
& K in our recipes, and these materials have extremely low solubility.
In some recipes, the need to lower expansion/conraction of a
fired glaze requires that some Na &/or K be replaced with Li feldspar or
Lithium Carbonate. If the substition is modest, the recipe will yeild a
satisfactory fired glaze; however, if too much Li compound is used, then
shivering will occur (the glaze contracts too much).
as for the alkaline earths: Calcium compounds are the standard,
and give solid results in most cases. yet some special effects can be
achieved with Ba compounds (eg, high Ba will cause copper to make a blue
glaze, not the usual green effect). and glaze mixes that depart from
glossy surfaces, that is, a matte surface is sought, then the matte effect
can be achieved by using extra Ca, or extra Mg, or subbing either of
these, with Sr or Ba. The difference between Ba & Sr is that Sr++ ions can
be handled by the body with little ill-effects should the fired glaze be
affected by materials that cause Sr++ ions to form and migrate to the food
we eat. This is not the case for Ba++ ions; if ingested as such the ions
may result in the body being poisoned.
as a basic rule, always use Ca materials in glazes for Cone 6 or
lower when making food-ware. some lesser amount of Mg materials or Sr
materials can be used in Cone 6 mixes and mixes of Cone 10/+. and
generally most potters accept the use of Ba materials only when the glaze
will go on surfaces that will not come into contact with food.
til later. peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
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David Beumee on sat 9 sep 06
Tom Buck wrote:
" In some recipes, the need to lower expansion/conraction of a
> fired glaze requires that some Na &/or K be replaced with Li feldspar or
> Lithium Carbonate. If the substition is modest, the recipe will yeild a
> satisfactory fired glaze; however, if too much Li compound is used, then
> shivering will occur (the glaze contracts too much)"
Wouldn't the lowering of thermal expansion by the use of lithium compounds mean possible shivering because the glaze would contract too little in relation to the clay body? Isn't the point of your paragraph to lower, rather than to raise, the contraction rate of a glaze?
David Beumee
-------------- Original message ----------------------
From: Tom Buck
> Kathryn Hughes:
> Perhaps you may benefit by obtaining a copy (via the WWW) of the
> Periodic Table of The Elements. It would show that the elements of Group I
> include the alkali metals: Lithium (Li), Sodium (Na), Potassium (K),
> and others. And Group 2 lists the alkaline earth metals: Beryllium (Be)
> (not used in claywork), Magnesium (Mg), Calcium, (Ca), Strontium (Sr),
> Barium (Ba), and Radium (Ra) (not used).
> Basically, the Table assembles similar elements in a Group, going
> from low atomic weight, to high atomic weight. So, within modest limits,
> an element low in weight can sub for an element higher in weight, or the
> other way around, high for low.
> Both Na & K are common is our daily lives: NaCl (common table
> salt), KCl (potassium chloride which is used medically when a patient is
> consuming too much NaCl and must lower its consumption). Sodium carbonate
> (soda ash) is often encountered, and it produces a highly alkaline
> solution in water, quite similar to lye (NaOH, sodium hydroxide).
> Potassium behaves similarly. So it shouldn't be a surprise that Lithium
> compounds will behave in a like fashion, although Lithium Carbonate (the
> compound often used by potters) has low (but not zero) solubility when
> used in a recipe and then water is added to make the glaze mix (a slurry).
> If the slurry is left standing for some days/weeks, it could be come quite
> alkaline and harm flesh. This doesn't happen with Na & K materials because
> (other than soda ash or potash) we use feldspathic materials to source Na
> & K in our recipes, and these materials have extremely low solubility.
> In some recipes, the need to lower expansion/conraction of a
> fired glaze requires that some Na &/or K be replaced with Li feldspar or
> Lithium Carbonate. If the substition is modest, the recipe will yeild a
> satisfactory fired glaze; however, if too much Li compound is used, then
> shivering will occur (the glaze contracts too much).
> as for the alkaline earths: Calcium compounds are the standard,
> and give solid results in most cases. yet some special effects can be
> achieved with Ba compounds (eg, high Ba will cause copper to make a blue
> glaze, not the usual green effect). and glaze mixes that depart from
> glossy surfaces, that is, a matte surface is sought, then the matte effect
> can be achieved by using extra Ca, or extra Mg, or subbing either of
> these, with Sr or Ba. The difference between Ba & Sr is that Sr++ ions can
> be handled by the body with little ill-effects should the fired glaze be
> affected by materials that cause Sr++ ions to form and migrate to the food
> we eat. This is not the case for Ba++ ions; if ingested as such the ions
> may result in the body being poisoned.
> as a basic rule, always use Ca materials in glazes for Cone 6 or
> lower when making food-ware. some lesser amount of Mg materials or Sr
> materials can be used in Cone 6 mixes and mixes of Cone 10/+. and
> generally most potters accept the use of Ba materials only when the glaze
> will go on surfaces that will not come into contact with food.
>
> til later. peace. Tom B.
>
>
> Tom Buck ) -- primary address.
> "alias" or secondary address.
> tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
> mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
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