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The Use of Barium

The Use of Barium in Clay Bodies



Section: Materials, Subsection: Safety

Description

Hazards of barium carbonate, considerations regarding its useinclay bodies for precipitation of soluble salts

Article

MSDS sheets indicate that barium (CAS: 7440-39-3) ispoisonand toxic if ingested. MSDS sheets also claim that barium isanirritant for inhalation and skin contact. Excessive amountscancause violent diarrhea, convulsive tremors, and muscularparalysis.Barium is known to affect the heart and nervous system.

In ceramics barium is most commonly inhaled as a fine dustorabsorbed through cuts or lesions on the hands of people whohandlecompounds containing the material. However, some authoritiesquoteexposure limits that are less restrictive than those forsilica(e.g. ACGIH lists a Threshold Limit Value at 0.5 mg/cubicmeter forbarium while quartz is 0.05). OSHA, on the other hand,lists quartzas 10.0 and barium as 0.5. (see the links to learn moreabout TLV,PEL).

Almost all raw clay materials (including kaolins) containsolubleimpurities (i.e. iron stained calcium/magnesium sulphates).When araw piece of clay is dried, these impurities come to thesurfacewith the water as it evaporates. During firing this oftenironstained precipitated scum left on the surface forms aglassydiscoloration and in some cases can even creates aglaze-likesurface.

Soluble salts are highly prized in sculpture bodies becausetheyhighlight surface textures and contours. This is especially soifthe salts are iron stained and create variations incoloration.However, in functional pottery and structural productsthese saltsare not wanted. Even after a clay is fired, thesesoluble salts cancome to the surface if the clay is not vitreousand it is rewettedoften (this is often the case with red terracotta brick, the brickindustry calls this phenomenonEfflorescence). These surfacedeposits often affect adherence to theclay and melting patterns ofoverlying glaze layers. Fused solubleson the surface can alsostick ware together or to kiln shelves andthey can seal thesurface causing premature bloating during firing.

Clay body and material supply companies commonly addbariumcarbonate to clay bodies in small percentages (0.1-0.8%) tosolvethis problem. The barium chemically reacts with the sulphatestoprecipitate insoluble products. In the reaction, veryslightlysoluble barium carbonate (in contrast with barium sulfateused inmedical x-rays which it is insoluble, inert,andnon-radioactive) and soluble calcium sulphate converttoinsoluble barium sulphate and calcium carbonate. Thus theybothremain within the body and do not concentrate on the surfaceduringdrying. The ceramic industry has found this strategy tobeeffective and has not found a viable substitute. It is possibletomeasure the amount of solubles in a clay body and calculatetheminimum amount of barium needed to precipitate them. Howeverinpractical terms this does not work because different types andevenshipments of barium vary in their effectiveness and thenaturalsalt content within the clays varies greatly. It is farmoreeffective to simply observe the effect of various additionsandadjust periodically so that there is just enough to removethesolubles (barium is expensive and industry obviously does notwantto use more than is necessary).

Controversy Surrounding Toxicity

The presence of barium in clay bodies is sometimes thought tobea health hazard and there are pros can cons to this view. Ononehand, barium is so very slightly soluble that it is difficulttodemonstrate solubility of any material in a simple labexperiment(MSDS sheets list it as insoluble). Nevertheless,theprecipitation of dissolved calcium and magnesium salts in thebodycan only occur if dissolved barium is present. Thusyouneed to add alot of barium carbonate (in a relative sense) toabody to generate the small amount of dissolvedbariumneeded for the reaction. The undissolved barium remains inthecarbonate form and thus can be ingested or inhaled. In somecaseseven the less than 0.5% typically present could be an issueforhypersensitive people. However on the other side of theissue,quartz is completely pervasive in almost all ceramic glazesandclay bodies and any balanced view of safety should putprimaryemphasis on it. In addition lab tests to deducebariumcarbonate content in aged pugged clay bodies have shown amuchlesser quantity of unreacted barium than the slightsolubilitysuggests is possible. This means the issue cannot befullyunderstood by considering only the slight solubility ofbarium.Further, as noted above, permissible limits for barium canbehigher than those for silica, thus the conclusion thatclaycontaining barium is a highly toxic material isnotnecessarily true. An MSDS from BassTech Internationalclassifiesbarium as moderately toxic on inhalation, non-toxic oncontact orskin absorption, an eye irritant and toxic if ingested at57mg perkg of body weight (this would be over 1 kg of puggedclaycontaining the highest proportion of barium normally used).

Leaching Hazard When Used in Clay Bodies?

The practice of adding barium to clay bodies isalsoquestioned by some as posing a leaching hazard in functionalwaremade from barium-containing clay. However one must considerseveralfactors that indicate otherwise:

  • The barium is distributed within the clay matrix and averysmall fraction of that which is added actually presents itselfatthe surface.
  • The barium reacts with comparatively abundant fluxes andsilica(and is a flux itself) in vitreous clay bodies toforminsoluble silicate glasses within the body.
  • Functional ware is covered with glaze completely isolatingthebarium from food surfaces.
  • Recipes employing highly refined clays and minerals often donotrequire barium, however many are subjected to a wide varietyofchemical processes that also have a bearing on functional safety.
  • The soluble portion of the barium converts to insolubleandharmless barium sulphate after the body is wettedduringpreparation, theoretically the insoluble portion is not astoxic.
  • Barium carbonate makes it possible to make bodies usingnativenatural coarser-grained materials (which are inherentlysaferbecause they generate fewer of the minus 1 micronparticlesthat are fine enough to lodge in the lungs).

    The US Food and Drug administration does not have aleachingrequirement for barium. However, the US EPA has a standardfordrinking water of 2 mg/liter or less. This standard was settoprotect the population most at risk for heart effects - *****men.Testing can be done at Elemental Research Lab, 309-267WestEsplanade, North Vancouver, B.C. Canada V7M 1A5 604-985-0445(theytest many metals). Also at Kirby Health Center Lab, 71NorthFranklin Street, Wilkes-Barre, PA 18701 717-822-4278.

     

Authors

  • Tony Hansen (Owner)
     
Posted by Ceramic Articles Database