GB2394471A

GB2394471A - Method of making an item of pottery from slate particles and clay

Info

Publication number: GB2394471A
Application number: GB0222676A
Authority: GB
Inventors: Scott John Dutton; Victoria Anne Buxton
Original assignee: Digive Ltd
Current assignee: Digive Ltd
Priority date: 2002-10-01
Filing date: 2002-10-01
Publication date: 2004-04-28
Anticipated expiration: 2022-10-01
Also published as: GB2394471B; GB0222676D0; GB2422601B; GB2422601A; GB0602505D0

Abstract

A method of making an item of pottery which includes the steps of making a generally homogenous wet mixture of slate particles, clay and water, the slate particles all having a maximum dimension of less than 3mm, to provide one of a formable composition and a pourable composition, the method including one of, where the composition is formable, forming the wet mixture to a desired configuration for the pottery item and where the composition is pourable, pouring the mixture into a mould of a desired configuration for the pottery item, and then at least partially drying the formed mixture as necessary to leave a residual water content of less than 2%, and firing the dried formed mixture to a temperature of at least 1000{C for a time sufficient for at least some of the clay to convert into a binder to bind together the slate particles, the slate particles being at least 50% by weight of the item and the clay being between 12 and 35% by weight of the item.

Description

239447 1

Title: Method of Making An Item of Pottery Description of Invention

['his invention relates to a method of making an item of pottery by which we mean an item of hollowware or flatware.

Traditionally items of pottery are made by moulding by hand or machine, wet clay, and then firing the moulded mixture. Decorations and glazes may be applied as desired.

Traditionally, for pottery items made of clay, prolonged firing times are required to heat the clay slowly to the maximum firing temperature to retain the integrity of the clay item. This is expensive in terms of time and energy.

According to the first aspect of the present invention we provide a method of making an item of pottery including the steps of making a generally homogenous wet mixture of slate particles, clay and water, the slate particles all having a maximum dimension of less than 3mm, to provide one of a formable composition and a pourable composition, the method including one of, where the composition is formable, forming the wet mixture to a desired configuration for the pottery item and where the composition is pourable, pouring the mixture into a mould of a desired configuration for the pottery item, and then at least partially drying the formed mixture as necessary to leave a residual water content of less than 2%, and firing the dried formed mixture to a temperature of at least 1000 C for a time sufficient for at least some of the clay to convert into a binder to bind together the slate particles, the slate particles being at least 50% by weight of the item and the clay being between 12 and 35% by weight of the item.

By making the item using predominantly slate particles rather than predominantly clay, it has been found that the wet mixture can readily be

formed and fired in shapes as intricate as can be produced conventionally in clay, and that importantly, the method presents the potential for firing the item considerably more quickly compared to an item made in clay.

The slate raw material is readily available as a waste product and thus is an inexpensive raw material. The fired item can be decorated and glazed in the same way as a conventional clay pottery item, as desired.

Desirably, the residual water content of the dried formed composition prior to firing is I/: to 1/% by weight or less, of the formed composition.

Preferably the maximum dimension of the slate particles is less than 2mm but a majority of the slate particles are even smaller than this.

The slate particles may include a mixture of particles including coarse particles having a maximum dimension of about 2mm, slate sand the particles of which have a maximum dimension of 0.4mm and slate powder the particles of which have a maximum dimension of 0.2mm.

Preferably about 2-10% of the solids of the mixture are coarse particles, about 6-15% of the solids of the mixture are slate sand particles, and the remainder of the mixture is slate powder.

Conveniently to facilitate making a generally homogenous mixture, the clay to be mixed with the slate particles is at least predominantly clay powder which is all able to pass through a size 85's lawn sieve having a mesh size of about 0.2mm. The clay may be ball clay, or any other desired kind of clay.

Particularly when the method includes making a formable composition, the constituents of the fired item may contain only slate and clay or may contain trace additives, for example to facilitate the applying of decoration or glazing subsequently. Where the mixture is pourable, a deflocculent such as sodium silicate may be added to the wet mixture to assist the mixture to flow more easily without thinning it. Such deflocculent is an electrolyte and suspension agent, and preferably accounts for less than 2% of the weight of the wet mixture.

Other materials may be added to the constituents of the formable or pourable compositions to improve the mechanical properties of the mix and the fired item. For example, fired clay (clay grog) may be added, and/or particles of Feldspar and/or Whiting but such added constituents preferably account for 10% or less by weight of the solid constituents of the mixture.

The formable wet mixture may include between 0.5% and 20% by weight water, the amount of water used depending on the proportion of and size of larger particles in the mix and the volume of the mix, so as to form a wet mixture which may conveniently be formed to the desired configuration which will retain its formed configuration well.

Preferably the formable wet composition is formed to the desired configuration by moulding in a mould by applying pressure.

The pourable composition may include up to 50% by weight, water.

The pourable or formable composition may naturally dry to leave a formed green-state pottery item. The composition may naturally dry to realise a residual water content of less than 2% by weight or may be dried by the application of heat. For example, the green-state item may be subject to a temperature gradient in a kiln, the temperature increasing towards a maximum firing temperature partially to dry the green-slate pottery item.

If desired, the partially dried green-state pottery item may be fired to a maximum firing temperature in excess of 1000 C e.g. at least 1100 C, preferably 1135 C and more preferably 1145 C. The higher the temperature, the faster the firing time, but preferably during firing the item is not subjected to temperatures in excess of 1200 C at which the configuration of the slate may be damaged.

In each case the solid constituents of the formed composition prior to firing may include between 15% and 35% by weight, preferably between 25% and 30% by weight clay powder, and between 65% and 85% by weight slate particles.

According to a second aspect of the invention we provide a generally homogenous mixture of slate particles and clay, for use in making an item of pottery according to the first aspect of the invention, the slate particles all having a maximum dimension of less than 3mm, and the clay being powder, the slate particles being at least 50% by weight of the mixture, and the clay between 15 and 35% by weight ofthe mixture.

Several examples of the invention will now be described with reference to the accompanying drawing which is an illustration of a firing curve.

Example 1

First, a homogeneous mixture (mixture A) of Fullersite slate (300g) and clay (60g) was prepared, with the slate thus accounting for about 83% of the dry composite mixture, and the clay for about 17%. The slate was in the form of powder particles, all of which were able to pass through a size 85's sieve having a mesh size of about 0.2mm.

The clay was ball clay in the form of a powder, all of which was able to pass through a lawn sieve of size 85's having a mesh size of about 0.2mm.

lithe composition was wetted until the water content of the mix was about 15% and the resultant mixture was formable by hand or by moulding into any desired pottery shape. The mixture was preserved in this state of wetness until formed. To remove any air present in the mix, the mixture was formed to a desired pottery shape (an item of hollowware) under pressure in a mould. After moulding and preferably after some drying to increase the strength of the green-state formed composition, the green- state pottery item was turned out of the mould.

After forming, before or after turning out of the mould, the formed greenstate pottery item was allowed partially to dry, for example while awaiting firing in a kiln or in the kiln as part of a firing cycle, in each case at temperatures lower than the maximum firing temperature. During such partial drying the water content of

the mixture reduced to less than 2%, by weight of the formed composition, preferably less than this, typically A% to 1%, or even less.

The partially dried green-state pottery item was then fired in an electric kiln for a relatively short time with the maximum firing temperature in excess of 1000 C being sustained for an hour or so, this time being sufficient for at least some of the clay to convert into a binder to bind together the slate particles. The resultant fired item was dimension for dimension, as strong as an item of earthenware produced using a conventional clay only mixture, but which required much longer overall firing times.

In the figure, there is indicated a firing curve. The green state item was heated over a period of about 5.5 hours to 1000 C, as indicated in region A of the curve. This temperature was sustained for about an hour at temperatures in excess of 1000 C as illustrated at B. Thereafter the item was allowed to cool (region C of the curve) and the overall firing time was about 6.5 hours.

Whereas an item of pottery was successfully produced using the method and mixture described, various other mixtures/firing temperatures and times were experimented with, some of which involved the addition of other materials to the mix to change the mechanical properties of the item of pottery produced, or merely to give a different texture of item. Examples of such materials are clay grog and/or Feldspar and/or Whiting up to about 10% by weight of the formable compositions.

Details of the solid constituents of these alternative mixtures which successfully produced items of pottery are given below.

Mixture A Slate (Fullersite) 300g (83.3%) (powder) Clay (powder) 60g (16. 7%)

Mixture E3 Slate (Fullersite) 300g (75%) (mainly slate powder with some coarse slate particles up to 2mm maximum dimension) Slate Sand 40g (10%) (maximum dimension of 0.4 mm so able to pass through a size 30's lawn sieve) Clay (powder) 60g (15%) Mixture E Slate (Fullersite) 400g (75.75%) (as mixture B) Clay (powder) 80g (15.15%) Clay grog (fired clay) 48g (9.1 %) (maximum dimension of 1.5mm so able to pass through a size 8's lawn sieve) Mixture F Slate(Delabole) 500g (80.6%) (mainly slate powder with some coarse slate particles up to 2mm maximum dimension) Clay (powder) 120g (19.4%) Mixture H Slate(Delabole) 400g (66.6%) (es mixture F) Slate Sand 50g (8.3%) Clay (powder) 150g (25.1 %)

Mixture J Slate(Delabolc) 500g (71.4%) (es mixture F) Clay (powder) 220g (28.6%) Mixture K Slate(Fullersite) 500g (71.4%) (es mixture B) Clay (powder) 220g (28.6%) Mixture M Slate (Delabole and/or Fullersite) 500g (65%) (as mixture B or F) Clay (powder) 200g (26%) Clay grog (fired clay) 70g (9%) (maximum dimension 0.8mm so able to pass through a lawn sieve size 20's) Mixture N Slate(Delabole) 500g (66.6%) (es mixture F) Slate Sand 50g (6.8%) Clay (powder) 200g (26.6%) Mixture O Slate (Fullersite) 500g (65%) (powder) Slate Sand 50g (6.4%) Coarse Slate particles 20g (2.6%) (maximum dimension 1.5mm to 0.8mm so able to pass through lawn sieve size 8's but not able to pass through lawn sieve sizes smaller than 20's)

Clay (Hyplas ball clay) 200g (26%) (in the form of powder) Mixture P Slate (Fullersite) 500g (60 6%) (powder) Slate Sand 55g (6.7%) Coarse Slate particles 20g (2.4%) Clay (powder) 250g (30.3%) Mixture R Slate (Delabole and/or Fullersite) 500g (63.4%) (powder) Slate Sand 50g (6.3%) Coarse Slate particles 20g (2.5%) Clay (powder) 220g (27.8%) In each of the above, the slate particles whether Fullersite or Delabole slate, were all able to pass through a lawn sieve of mesh size 85's (0.2mm), and thus had a corresponding maximum dimension. The clay powder was ball clay but other types of clay may be used, the clay powder all being able to pass through a sieve of mesh size 85's The mixture of slate and clay etc. was mixed with water to bring the water content up to between 0.5 and 19%, to result in a formable composition.

The mixtures were moulded as described above for mixture A and fired to a maximum firing temperature in excess of 1000 C (typically 1135 C) but which did not exceed 1200 C as temperatures in excess of 1200 C can result in

the slate particles expanding and thus being damaged. The mixtures were fired for between 3 and 6.5 hours, nominally 5.5 hours.

In the fired item of pottery, for each mixture the slate particles accounted for at least 50% by weight of the item, and preferably at least 60%, and the clay accounted for at least 12% and preferably at least 17% by weight of the item.

Example 2

A wet mixture of slate particles of maximum dimension 2mm and clay powder of maximum dimension 0.2mm, and sodium silicate was prepared with water. The water accounted for more than 50% by weight of the wet mixture, the slate for more than 25%, and the clay for about 15%. The sodium silicate accounted for less than 2% of the weight of the wet mixture.

The wet mixture was pourable and thus poured into a mould for a pottery item, i.e. an item of hollowware.

The mixture was allowed naturally to settle and partially to dry by evaporation with water settling out of the mixture being removed from the mould as desired, until the water content had reduced to about 15-19% by weight, when the mixture will have dried to a green state.

Then the moulded green state pottery item was removed from the mould and partially dried to leave a water content of less than 2% by weight and fired, as with example 1.

In the firing curve shown in the figure, there is indicated at region D, a reduction zone. During this part of the firing curve, oxygen was removed from the kiln. This hastened firing, but the object was to produce pottery items of different colour. In another example, gases may be added to affect the colour of the fired item. In a gas kiln, oxygen reduction may be achieved as air is used up in the kiln for combustion.

The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process

for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A method of making an item of pottery including the steps of making a generally homogenous wet mixture of slate particles, clay and water, the slate particles all having a maximum dimension of less than 3mm, to provide one of a formable composition and a pourable composition, the method including one of, where the composition is formable, forming the wet mixture to a desired configuration for the pottery item and where the composition is pourable, pouring the mixture into a mould of a desired configuration for the pottery item, and then at least partially drying the formed mixture as necessary to leave a residual water content of less than 2%, and firing the dried formed mixture to a temperature of at least 1000 C for a time sufficient for at least some of the clay to convert into a binder to bind together the slate particles, the slate particles being at least 50% by weight of the item and the clay being between 12 and 35% by weight of the item.

2. A method according to claim 1 wherein the residual water content is between i/: to 1/% by weight, of the formed composition.

3. A method according to claim 1 or claim 2 wherein the maximum dimension of the slate particles is less than 2mm.

4. A method according to any one of the preceding claims wherein the slate particles include a mixture of particles including coarse particles having a maximum dimension of about 2mm, slate sand the particles of which have a maximum dimension of about 0.4mm and slate powder having a maximum dimension of 0.2mm.

5. A method according to claim 4 wherein about 2-10% of the solids of the mixture are coarse particles, about 6-15% are slate sand particles and the remainder is slate powder.

6. A method according to any one of the preceding claims where the clay to be mixed with the slate particles is at least predominantly clay powder which is able to pass through a size 85's lawn sieve having a mesh size of about 0.2mm.

7 A method according to any one of the preceding claims wherein the method includes making a formable composition, the constituents of the fired item containing only slate and clay.

8. A method according to any one of claims 1 to 6 wherein the mixture is pourable and a deflocculent is added to the wet mixture to assist the mixture to flow more easily without thinning it.

9. A method according to claim 9 wherein the deflocculent is sodium silicate and accounts for less than 2% of the weight of the wet mixture.

10. A method according to any one of claims l to 7 wherein the formable mixture includes between 0.5% and 20% by weight water.

11. A method according to any one of claims l to 7 and 10 wherein the mixture is lonnable and the wet composition is formed to the desired configuration by moulding in a mould by applying pressure.

12. A method according to any one of claims 1 to 6, 8 or 9 wherein the mixture is pourable and the pourable composition includes up to 50% by weight, water.

13. A method according to claim 12 wherein the pourable composition naturally dries to leave a formed green-state pottery item.

14. A method according to any one of claims 1 to 13 wherein the green state pottery item is subjected to a temperature gradient in a kiln, the temperature increasing towards a maximum firing temperature partially to dry the green-

slate pottery item.

15. A method according to any one of the preceding claims wherein the green-state pottery item is fired at a temperature in excess of 1000 C e. g. at least 1100 C, preferably 1135 C and more preferably 1145 C but less than 1 200 C.

16. A method of making an item of pottery substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

17. A generally homogenous mixture of slate particles and clay, for use in making an item of pottery according to the first aspect of the invention, the slate particles all having a maximum dimension of less than 3mm, and the clay being powder, the slate particles being at least 50% by weight of the mixture, and the clay between 15 and 35% by weight ofthe mixture.

18. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.