CA1113660A - Bone ash - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A bone ash for the manufacture of bone china comprises calcined bone on which has been incorporated by calcination a source of B2O3. The incorporation of the B2O3 in the bone ash facilitates the manufacture of bone china, particularly promoting the formation of the micro-crystalline phase which is characteristic of bone china, thereby making it possible for bone china to be made with a wider firing temperature range, in a shorter time and if desired, lower temperatures than normal.

Description

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This invention relates to a novel boron-containing bone ash and method of rna]cing it, for use in the production of bone china.
This application is a division o~ Canadian Patent Application S.~. 316,350, filed November 16, 1978.
~' The effect of using a boron-containing material or substance as flux in making bone china is to promote formation of the microcrystalline phase which is characteristic of bone china, and thus make it possible for bone china to be made with a wider firing temperature range, in shorter time and/or at lower temperatures than normal.
Bone china is made by firing a mixture of clay, bone ash and a flux. Normally the clay is predominantly kaolin or ir ' china clay. The most commonly used flux is Cornish Stone but other naturally-occurring felspathic materials such as ~` nepheline syenite may be used.
,il Bone ash such as used in the production of bone china ,~ is made by first of all treating animal bone with hot or boiling ~ water so as to remove gelatine, collagen and other organic , '~ 20 matter. In this state the bone is said to be de-gelatinised. It is afterwards calcined to a temperature of about 1000C. in ~` order to burn off the remaining organic matter. Calcined bone or bone ash is substantially (about 80 percent) calcium phos-. . , pha-te, with some calcium carbonate and alittle fluoride.
` Ceramic compositions for making bone china comprise ~; typically 50% bone ash, 25% china clay and 25% Cornish stone.
They are made into a ceramic body by intimately mixing and usually wet milling the particulate ingredients so as to pro-duce an aqueous slurry or slip suitable for casting in plaster moulds; or at lower water content a plastic clay which may be moulded or shaped. After drying to evaporate the water such '.

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shaped objects in -the `'green" state are then fired to a tempera turn in the region of 1250C. in order to produce unglazed `'china biscui^t" ware of low porosity and characteristic trans-lucency. The biscuit ware is subsequently coa^ted with a glaze slip and re-fired to a temperature of about 1100C. to produce glazed china. One ofthe principal characteris^tics of ware produced in this way is the translucency, coupled with white-ness, for which bone china (sometimes called fine china or English bone china) is renowned. Because of its strength, it is also possible to produce objects of thinner wall section than is the case with other forms of pottery, which enhances lightness of weight, translucency and resistance to thermal shock.
The final microstructure of bone china results from reactions during firing, the crystalline materials originally produced having been dissolved and new crystals formed. Bone china after firing consists of about 70% crystals and 30% of a ` glassy phase. The crystals are mainly ~-tricalcium phosphate and anorthite (a lime felspar). m e high crystal content of bone china accounts for its good mechanicalstrength.
Bone china of substantially the above type and composition has been produced for some two hundred years-and changes in formulation and fabrication have been evolutionary.
The composition requires a higher firing temperature than is needed for earthenware and consequently the firing cycle is comparatively prolonged and the costs of both the kilns and the fuel used are higher. The firing time is also lengthened relative to earthenware in order to allow formation of the calcium phosphatle-based phases which impart the characteristic .
strength and translucency.

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Methods fox mciking the ceramic clayware called "white ware", from clay, flint and a flux, in which certain boron-containing compositions are used as the flux are known. According to U.S. Patent 3,53~,522, a borate other than an alkali metal borate is used as the flux, e.g., calcium borate. According to U.S. Patent 3,704,146 a ground vitreous frit which includes B203 in its oxide analysis is used as the flux.

By means of the present invention, the formation of the calcium phosphate micro-crystalline phases is enhanced by the inclusion of a boron-containing substance. The boron-containing substance can serve as the flux or as part of the flux. Such inclusion aids vitrification and enables the china ware to be produced at a lower temperature or in a shorter time, or both.
A~ an added benefit, the temperature range for firing is widened. This practice reduces fuel consumption, enables simpler kilns to be used, speeds firing rate and helps to avoid distortion of ware by virtue of the wider temperature ranges, as well as lower temperatures, required.

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The boron-containing subs-tance rnay be combined wi-th the bone-ash, it may be either one of the inherently water-insoluble borates or fluxes described previously as fluxes for whiteware, or a water-soluble boric oxide containing material, but if water-soluble it must be bound to the bone-ash so as to be made insoluble. The boron-containing material may be bound to the bone ash by being fused or sintered or calcined with calcined bone, or by being calcined with de-gelatinized bone so -that the calcination simultaneously converts the bone to bone ash and binds the boron-containing material to it.
The boron-containing substance may be boron phos-phate, an inorganic borate other than an alkali-metal borate (e.g., a calcium borate), or may be a ground vitreous frit whose oxide analysis includes SiO2, B203, CaO and/or MgO, and Na20 as described in U.S. Patent 3,704,146 or British Patent 1,254,717. Such materials may be used in the process of this invention either by milling them into the slip (or plastic clay) together with bone ash, kaolin and optionally Cornish stone, or by combining them with bone or bone ash - as is necessary for water-soluble boron-containing materials.
`~ The boron-containing material or substance may alternatively be a borated bone formed by reaction of bone with boric acid or one of its water-soluble salts.
For example, boric acid or its salts '' .~

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can be combined with th~ bone ash by being fu~ed or sintered wi-th the ash, and the resultant rnaterial incorporated as pulverulent material in the 91ip. ~n alternative method of combination is to absorb an aqueous solution of the B203-containing material (or an aqueous suspension of a water-însoluble ~oron-containing mate-rial) onto bone, preferably after the bone has been deyelati-nised and before it is calcined to make bone ash, and then calcine the bone and adsorbate or absorbate. Pre~erred water-soluble boric oxide-providing materials are boric acid, sodium borate of Na20:B203 ratio 1:4, and borax (sodium tetraborate decahydrate).
Calcination of bone containing boric acid or similar material is not always sufficient to make the B203 completely water-insoluble, and it has been found that insolubilization can be improved by using also the precursor of at least one additional glass-forming oxide, e.g., sodium silicate, potassium carbonate, ; aluminum sulphate and calcium hydroxide to provide ~a20, SiO2, K20, A1203 and CaO respectively as the additional gLass-forming oxides. Alternatively, or additionally, boron phosphate can be used as an insoluble boron-containing flux. This may be added in the form of prepared boron phosphate - formed in situ by adding an equimolar mixture of boric and phosphoric acids to the bone before calcination.
The bone fragments steeped in aqueous li~uor containing the source of B203 and optionally of the additional oxide are dried and then calcined at a temperature sufficient to react with or simply to form a glassy layer on the surfaces of the-bone.
It is not essential for the glassy layer to interact fully at this stage with t:he calcium phosphate substrate but desirable for the borate glass formed to be sufficiently insoluble in water after grinding for casting slips or mouldable clays to ~e-made satisfactorily on the addition of clay and other ingredients.
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1 Alternatively, pre-caLcined bone may also be steeped in borate-~ontaining liguors, dried and re-calcined to insolubilise the absorbed or adsorbed salts prior to milling. It is well-known that bone ash calcium phosphate has a high surface area and 5 therefore a high liquid absorption capacity, but it has been found that th~ porosity and internal surface area of de-gelatin-ised but uncalcined bone are adequate to accept sufficient boric acid or borate salt solution for the practice of this invention.
The advantage of using de-gelatinised bone is that only one cal-10 cination and milling operation is necessary to prepare a satis-factory ceramic body ingredient, thus effecting cost and fuel savings.

When boric acid (or B~03 or metaboric acid) alone is 15 used as the B203 source and no other glass-forming oxide pre-cursor is introduced, the bone reacts on calcination to form CaO-P205-B203 phases. Compositions at all possible ratios with-in this ternary phase system do not appear to have been studied, but there is described in the literatur~ a calcium borophosphate 20 of composition 2CaO P205~B203 which is decomposed by water.
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- Calci~m borophosphate compositions have been found to disproportionate on heating to high temperature. For example, the composition 2CaO-P205~B203 melts incongruently at aroun~
25 10~0C. to form ~-2CaO P205 and li~uid B203 Related compositions ; within this ternary system behave similarly and it is believed that the effect of a calcium phosphate phase crystallisation and synerisis of a B203-containing liquid phase also occurs in compo-sitions to whiich Na20 has been introdllced initially. During the 30 firing of conventional bone china, the bone phosphate; kaolin and Cornish stone interact to form glassy phases from which ~-calcium phosphate and anorthite crystallise subseguently.

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otal melting ~oes not occur an~ the more refractory phases remain and help to retain the shape and relative riyidity of the object being fired. It is found -that the interaction of particulate ingredients is accelera-ted by the presence of B203-containing substance in the calcined bone. This enables a corresponding and satisfactory degree of vitrification to be achieved, without distortion of shape, at lower temperatures and in shorter firing schedules. The mechanism is not fully understood but -the liberation of B203-rich phases and their reaction and bonding with the siliceous ingredients is likely.
The microcrystalline phases remaining after cooling are pro-bably similar tothose contained in conventional bone china but their crystallite size and shape, and the proportions of respective crystalline phases may be different.
On calcination to a suitable temperature, insolu-bilisation of the initially highly soluble B203 substance such as sodium borate takes place by chemical reaction with the calcium and phosphate contents of the bone.
Thus, one aspect of the invention is a bone-ash for ~ 20 the manufacture of bone china comprising calcined bone on - which has been incorporated by calcination a source of B203.
In another aspect of the invention there is pro-vided a method for preparing bone ash for the manufacture of bone china which comprises degelatinizing animal bone and calcining said degelatinatized bone to produce bone ash, the improvement which comprises incorporating B203 in said bone ash to produce a boronated bone ash in which said B203 is in water-insoluble form.
In a particular embodiment of the invention there is provided a method of preparing bone ash, which comprises degelatinizing animal bone, steeping the fragments of the bone ash in an aqueous solution or suspension of a B203-' :

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containing substance, whereby B203 or its precursor is absorbed or adsorbed on-to the bone, and calcining the bone to convert it to bone ash.
Thus, a borate flux can be introduced in fine parti-culate form to the bone before or during calcination, so that during the final stages of this calcination step the calcium phosphate and B203-containing material begin to interac-t.
In another embodiment of the invention there is provided a method of preparing fluxed bone ash for the manu-facture of bone china, which comprises degelatinizing animal bone, mixing the degelatinized bone with a substance which when calcined ma~es available B203 for combination with the bone or which contains B203 and when calcined combines with the bone, and calcining the mixture to convert it to boron-containing bone ash.
Still fur-ther the invention provides a method of preparing bone ash, which comprise~ degelatinizing and cal-cining animal bone, to produce bone ash, steeping the bone ash in an aqueous solution of B203 or a substance which on heating is converted to B2Q3 whereby B203 or its precursor is absorbed or adsorbed onto the bone ash, and calcining the bone ash.
There is disclosed a method of making bone china, which comprises forming a slip containing clay and bone ash containing B203, particularly about 1 to 12%, preferably 2 to 8%, by weight of B203, shaping the slip to a body and firing the body.
There is also disclosed the method of making bone china by firing a composition comprising bone ash and clay, the improvement which consists of adding to the composition prior to firing a boron-containing substance which is in water-insoluble form.

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S-tlll further there is disclosed a bone china corn-prising a fired composition of clay and bone ash, in which the bone ash contains B203 and which can be produced by the afore-mentioned me-thod.
The amount of B203-containin~ substance incorporated in the bone-clay mix-ture should be sufficient to provide from about 0.5 to about 6% by weight of B203 in the composition, preferably about 1 to 4% B203. ~hen borated bone is employed, the bone should have about 1 to about 12% B203 and preferably 10 about 2 to 8% B203 content.

The followiny examples illustrate the invention:
Exam~le I
Preparation of Borated BOne Four bone samples, reference A, B, C and D in Table I, were prepared from bone which had been de-gelatinised and calcined, and five samples, reference E, F, G, H and J in Table I were prepared from bone which had been de-gelatinised ~; but not calcined. Samples A and E were treated with aqueous boric acid solution to introduce the quantity of B203 shown in ; 20 Table I, Samples B and F were treated with an aqueous solution of sodium borate (1:4 Na20:B203) to introduce the quantities of Na20 and B203 shown in Table I, Samples C and G were treated with an aqueous solution of borax to introduce the quantities of Na20 and B203 shown in Table I, Samples D and H were treated with an aqueous solution of borax and sodium silicate to intro-duce the quantities of B203, ~a20 and SiO2 shown in Table I, and Sample J was heated with an aqueous solution of boric and phos-phoric acid= to intrcduce the quantities shown in Table I.

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TABLE I

A B C D E F G H J
(Darts ) calcined bone 90 88 ~6.4 78.g degelatinised bone .95 88 87.6 80.1 100

2 3 10 9.8 9.4 8.6 S 9.8 8.6 8.6 ; 3 3 : Na2O 2.2 4.2 g.6 ~ 2.2 3~8 8~8

3 4 9.2 sio2 - 2.9 2~5 For each sample, the bone was immersed in the aqueous . . . solution, dried and then calcined at 1000C. After calcination, : Samples A to D, which had been made from a calcined powder, were lightly sintered and these were re-ground to pass 350 B.S. mesh.
After calcination samples E to J, which were in the form of small ; . lumps o treated bone, were broken down and ground to pass 350 B.S. mesh.
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2~ Example II

. Ceramic Compositions ' , . . '' '''' ' '' ' Each of samples A to J from Example I was used as a-~ 25 ;~ component in a ceramic mix, and two samples of calcined but unborated bone were similarly used, as a con~rol, in a ceramic mix. Compositions of the eleven mixes were as-in Table II.

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1 The dry powders were thoroughly mixed and compressed into rods lf2 inch diameter x 1 inch in leng-th. They were placed in a kiln and the temperature xaised to 1050~C, at a rate of lOODC./hour, maintained at 1050C. for 6 hours and slowly cooled ~o ambient temperature. In addition, specimens of the ; "standard bone china" mix (control 2) were heated to 1250~Co~
the normal firing temperature for bone china.
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Compression strength measurements were carried out on the specimens to indicate the degree of vitrification. The results were as in Table III.
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- 15 : ~ -Firing Compression Times Stronger Temperature ~C~) ~trength (psi) than Control 2 - Control 1 :1050 480 - .
Control 2 1250 10700 ; ; A 1050 13400 - 1.3 - B ~ 12~00 1.2 C ~ 15800 1.5 `D .. n 12000 1.1 : E ~' 11000 1.0 ~: . F n 12400 ~ 1.2 " ~ 25 G n 14800 17 4 - .
: H ~ 9100 0,9 " 11000 1.0 . ~ .
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1 ~xample III

Ceramic Composition ~s an alternative to the use of bone in which a ' B2O3-cont~ining substance has previously been incorporated, it . .is possible to introduce B2O3 to the ceramic mix as a separate .': ' composition in admixture with the bone and other body ingredi-ents. In this example, a body made by milli.ng together 50 parts :'~' 10 of calcined bone, 10 parts of the borate flux "24 S50"''descr'ibed -. in Patent 3,707,146, 20 parts of china clay and 20 parts of :.' 'Cornish stone was ~ired ~or 6 hours at 1050DC. The fired body l' had a compression strength of 12600 p~s.i. which was 26 times ,:~
'. .'stronger than the control body (50 parts bone, 25 parts china ;
- 15 clay, 25 parts Cornish stone~ prepared and.fired under iaenti-.' cal conditions and was 1~2 times stronger than the control ' :: body fired at 1250C.
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; . . Example_IV
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This example shows that use of a separate flux is not essential when the borated bone of this invention is used. ~`.
100 parts of c~lcined bone was mixed with 20 parts anhydrous .
: 'borax and 10 parts silica. This mixture was calcined at 1000C.
25 `and ground to pass 350 mesh B,S. ' -.
'~ ' ' ' .~.' ' A body mix was prepared from 65 parts of this borated ~ bone and 35 part5 china clay~ pressed and fired at 1050C. as :~ ' in Example II. Fired bodies were of strength'equal ts that of ' ~~' '~ a control body fired at 1250C
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1 Example V

This exarnple shows that: bone china bodies having ade-quate strength and translucency may be prepared using the ~lux of this invention as the sole fluxing agent and firing to temperatures substantially lower than 1250C.
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Bodies were prepared by blunging the compositions in Table IV in water, removing excess water on a Plaster o~ Paris . 10 bat and extruding through a deaerating pugmill. Rods 1/2" dia-meter x 6" were prepared for measurement of translucency.
Modulus of rupture was measured hy measuring the load applied -. at the mid-point requlred to break a specimen supportea at

4 inch centers. Translucency was determined.by measuring the li~ht transmitted by the specimen and is e~pressed as a per-- centage of the amount of light of the same intensity transmitted ~ by standard bone china fired at 1250C. Results are given in : Table V.
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- TABLE IV
Composition Calcined China Benton- Borate* Borate* Boron No. Bone Clay ite 24 SS0 A S50 Phosphate 3 63 311 . S
. 4 64 321 3

5 . 64 321 3

6 65 321 2 *as described in U~ S, Patent No. 3,704,.146.

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3~0 1 l'ABLE V
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Composition Firing Modulus of Rupture Translucency . No. Temperature (~C.) (lb/sq.in,-)- -- (%) 1 1180 13,900 103 2 1215 1~,500 118 - 3 118~ 300 109 :: 4 1215 15~200 113 1180 18~g.00 47 6 125Q 17,000 102 ( Control 125012,000-15~000 ious changes and modi~ications of the invention can be m~de~ and~ to the extent that such variations incorporate the ~pi~it of the invention~ they are intended to be included within ~- the scope of the appended claims.
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~ 30 ~ ' , ' '~ . ' : : -SUPPLEME~r~RY DISCLOSURE
This disclosure and the Principal Disclosure are concerned with bone china, bone ash and their preparation.
As described in the Pr:incipal Disclosure the formation of calcium phosphate ~icrocrysta:Lline phases in the bone china can be enhanced, in accordance w:ith the invention, by the inclusion of a boron-containing substance which serves as a flux or as part of the flux, ~he boron containing substance or flux may be one of the inherently water-insoluble borates, frits or glasse~ or a water-soluble boric oxide containing material, as described or it may be a borated bone, water soluble salt or water-in-. soluble B203 containing material.
By means of the invention there can be produced a bone ash for the manufacture of bone china, in which the `: bone ash comprises calcined bone on which has been incorporated by calcination a source of B203. In particular there may be obtained a bone ash containing about 2.5 to 5% by weight B203.
Further, as described previously, the invention provides a method of preparing fluxed bone ash for the manu-facture of bone china, which comprises degelatinizing animal bone, mixing the degelatinized bone with a substance which ~ when calcined makes available B203 for combination with the - bone or which contains B203 and when calcined combines with . the bone, and calcining said mixture to convert it to boron-containing bone ash, As described in the Principal Disclosure one aspect of the invention is a bone-ash for the manufacture of bone china comprising calcined bone on which has been incorporated by calcination a source of B203. Another aspect of the invention is a method of preparing bone ash, which comprises degelatinizing animal bone, optionally (but preferably not) , .

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calcining the degelatiniYed bone, then mixiny the deyelatinized or calcined bone with a substance which contains B203 and then calcining t'he mixt~re whereby B203 or a precursor t'hereof is absorbed or adsorbed onto the bone. In this process, the source of B203 may be a wa-ter-soluble chemical compound of B203 in which case fragments of the bone are steeped in an aqueous solution or suspension oE said chemical compound. In the preferred form of the method of this invention, however, the source of B203 is a ground vitreous frit having B203 in its oxide analysis, and then a mere mixture of the degelatinized bone and the powdered frit can be calcined with satisfactory results.
If desired, the degelatinized bone can be mixed with a slurry of the ground frit in water to form a paste, which is then calcined.
The preferred flux is a ground vitreous frit which ~; includes B203 in its oxide analysis, namely a ground vitreous frit whose oxide analysis includes SiO2, B203, CaO and/or MgO, and ~a20. The frit may'optionally also include A1203, When ' 20 bone china is made by slip-casting, it is appropriate to deflocculate the slip and we have found that a borated bone ash made by calcining together degelatinized bone and such a ground vitreous frit, when combined with clay to form a slip fgives a slip which can be deflocculated effectively.
' For the co-calcination of bone with frit, a ; temperature in the range of from about 900 to 1200C~ is f ~, r `' suitable, and 950 to 1100C. is preferred. The quantity of ..
' the frit is generally small in relation to the quantity of bone, e.g., about l~/o by weight frit,-9~/O by weight bone.
The bone is suitably borated with the frit to contain about 1 to about 12% B203, preferably about 2 to ~/O with 2.5 to 5%
B203 being most preferred. This content may be achieved by ,, ~ ' - 17 -- .
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mixing a more highly borated bone with ordinary bone.
In a preferred rnethod of the invention, an aqueous slurry of a finely-ground (about 80% less than 25 microns) B203-containing frit is mixed with degelatinized bone, such that the particles of frit stick to the larger (up to about 3/8 inch) particles of bone, on the surface, in the pores, etc.
This mixtur~ is then calcined at a temperature of about 1000C.
cooled, dry-crushed and then wet-milled. The resultant slurry is filtered and the cake dried and crumbled or lightly milled.
This product can then be mixed with the appropriate amount of kaolin (and optionally a small amount of Cornish stone) and - water to give the casting slip from which the ware is made.
Although slip-casting is preferred, other known methods for manufacturing flatware may be employed with the compositions of this invention.
The following examples further illustrate the invention.
Exam~le_VI
A vitreous frit having the oxide analysis l~/o ~a20~ lOD/D
CaO, 25% B203, 5% A1203 and 5~/~ SiO2 was prepared by the method ~ described in British Specification ~o. 1,254,717 and ground ; to particles small enough to pass a 350 B.S. mesh.
725 g. of degelatinized bone (about 500 g. solid and 225 g. absorbed water) in the form of small lumps of average size about 1 cm. were mixed with 64.5 g. of the finely ground frit slurried in 60 ml. of water. The mixture was calcined at 1000C~ for 3 hours. The product was dry ground and wet milled to give a finely powdered, ready-fluxed bone containing ; 3.59% B203.
The fluxed bone was used to prepare a bone china body slip to the following formulation.
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Eluxed bone 69 g.
China clay 31 y.
Water 50 ml.
The mixture, aE-ter deflocculating with a poly-acrylate deflocculant, gave a low viscosity, free~flowing slip suitable for slip casting.
Example VII
(i) A frit as described in British Patent Specification ~o. 1,254,717, comprising 10.3% ~a20, 11,7%
CaO, 44,7% B203 and 33.3% SiO2 was prepared and finely ground. 725 g. of degelatinized bone (containing absorbed water as before) were mixed with 37.5 g. of frit slurried in 50 ml. of water. The mixture was calcined and milled as described in Example VI to give ready-fluxed bone containing 3.95% B203.
(ii) A ready-fluxed bone was prepared as described in (i) using a frit comprising 10.3% Na20, 11.7% MgO, 44.7%
B203 and 33.3% SiO2, ` The bone boronated both with the CaO-containing and - 20 with the MgO-containing frit was formed into a slip with china clay and water, and again gave a good slip for slip-casting, - ~x Example VIII
The compositions containing borated bone, clay and water according to Examples VI and VII can be de-watered to a pasty consistency. The resultant plastic clay, of lower water content than a slip, can be molded and shaped by known ~ .
techniques to make plates, saucers and other flatware.

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Claims (17)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:-

1. A bone ash for the manufacture of bone china com-prising calcined bone on which has been incorporated by calcination a source of B203.

2. A bone ash according to claim 1, in which the source of B203 is a ground vitreous frit whose oxide analysis includes B203.

3. A bone ash according to claim 2, in which the frit has an oxide analysis including B203, SiO2, Na20 and at least one of CaO and MgO.

4. A bone ash according to claim 1, in which the cal-cined bone includes also A1203.

5. A bone ash according to claim 1, 2 or 3, containing 2% to 8% by weight B203.

6. In the method for preparing bone ash for the manu-facture of bone china which comprises degelatinizing animal bone and calcining said degelatinatized bone to produce bone ash, the improvement which comprises incorporating B203 in said bone ash to produce a boronated bone ash in which said B203 is in water-insoluble form.

7. A method of preparing fluxed bone ash for the manu-facture of bone china, which comprises degelatinizing animal bone, mixing the degelatinized bone with a substance which when calcined makes available B203 for combination with the bone or which contains B203 and when calcined combines with the bone, and calcining said mixture to convert it to boron-containing bone ash.

8. A method of preparing bone ash, which comprises degelatinizing animal bone, steeping the fragments of the bone in an aqueous solution or suspension of B203 or a sub-stance which on heating is converted to B203, whereby B203 or its precursor is absorbed or adsorbed onto the bone, and calcining the bone to convert it to bone ash.

9. A method according to claim 8, in which aqueous boric acid or aqueous sodium borate is used as the source of boric oxide.

10. A method according to claim 8, in which said sub-stance is a ground vitreous frit whose oxide analysis includes B203.

11. A method according to claim 10, in which said frit has an oxide analysis including B203, SiO2, Na20 and at least one of CaO and MgO.

12. A method according to claim 8, in which A1203 is also introduced into said bone.

13. A method of preparing bone ash, which comprises degelatinizing and calcining animal bone, to produce bone ash, steeping the bone ash in an aqueous solution of B203 or a substance which on heating is converted to B203 where-by B203 or its precursor is absorbed or adsorbed onto the bone ash, and calcining the bone ash.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

14. A bone ash according to claim 2 or 3, in which the frit has an oxide analysis which also includes A1203.

15. A bone ash according to claim 1, 2 or 3, containing 2.5% to 5% by weight B203.

16. A method according to claim 10 or 11, in which the oxide analysis of said frit also includes A1203.

17. A method according to claim 6, 10 or 11, in which said B203 is present in said bone ash in an amount of 2.5%
to 5% by weight.