1. INTRODUCTION 

In making large ceramic objects, the plastic properties and weight of the material have traditionally limited modelling possibilities. Fragility and shrinkage in the green state have been frequently encountered problems with conventional claybodies. Clay reinforced with paper fibre, however, is an excellent material for large pieces, sculptures and slabs, because of its remarkable green strength and lightweight. In paperclay, the clay particles glue the paper fibres into a network and thus form a supporting structure for an unfired object and prevent cracking. The paper fibres burn away from the clay in firing, leaving the object porous.

Use of paperclay is economical. Instead of requiring new paper, all kinds of waste paper, recycled paper and pulp are good material to be mixed with clay. Material and energy costs are cut down as paper fibre added to clay body fills up the clay mass and contributes its heat value to firing (1). Paperclay also lasts rapid temperature changes, which shortens the firing time and saves energy.

The capability of cellulose fibre to absorb water is an essential advantage to claybodies in plastic state. Being hollow, cellulose fibre is very absorbent and withstands compression and twisting. Since clay particles are much smaller than fibres, they are absorbed to the surface of the fibre as the clay dries. The rough surface enables the fibre to be attached to the clay tightly enough so as not to slip from its hollow cavity.

Since the benefits of paperclay in comparison to conventional claybodies lie in its strong green strength and light fired weight, essential research issues have been bending strength, porosity and shrinkage and as bending in firing. The strength of paperclay is determined by clay composition and the paper fibre type, as well as the relationship between fibre amount and quality. Also the production method and firing temperature is significant. The purpose of this series of experiments was to study how much the fibre type affects the strength of the claybody and firing strength. The tests were carried out in cooperation with Helsinki University of Technology, and Finnish paper mills contributed with paper fibre samples.

The starting point of this study was the research on paperclay as sculpting material by Rosette Gault, of USA (2). Beside the material study, an artistic part is included in this study, where the practical applications of paperclay were tested. The firing temperature of 1100oC was focused, since the pieces were painted with ceramic pigments, some of which are burned off at higher temperatures.

2. ULTIMATE BENDING STRENGTH

The objective of this study was to find a paperclay composition where the fibres would yield strong green strength and where the porosity created by the fibres in firing would be smooth, delicate and evenly spread. In other words, the fired clay is lighter than the corresponding pure base claybody without sacrificing and technical properties essential. 

Paper and cellulose were reduced to pulp before mixing into the claybodies. The paper was first soaked in hot water and beaten to disintegrate the fibres. The excess water was squeezed out by using a large mesh screen to get a pulp with approximately 20 percent water, where the clay was added. The ratio of fibre varies from 2%, 10% and 20% of dry fibre in relation to the dry weight of the clay.

From the test material, 22 test batches were prepared: with seven different fibre types, three different fibre ratios and a base claybody with no fibre added. Ten test bars, each measuring 25 mm x 25 mm x 150 mm, were made of each test batch by pressing into a one-sided plaster mould. The clay was pressed by hand and the open surface was smoothened with a splint. The tests were repeated five times with both unfired pieces and pieces fired at 1100oC.

The tests were made at the Helsinki University of Technology at the metal laboratory with Zwick 1385 drawing engine as three point bending fatigue test, where the test bars were bent at a speed of 2 mm/min. The measurement method was adapted according to ISO 3327-198(E) specification. Test materials were different fibre types, selected for being easily available: waste paper, pulp, and rejects that develop in the paper manufacturing industry. The clay bodies were all earthenware.

Composition of clay body and paper fibre types.

Base clay body Ml                              Fibre types
25%: Ball clay Hyplas 71                   A: Pulp reject 
25%: Kaolin Grolleg                           B: Ground wood pulps reject 
10%: Silica                                           C: Fibre recovery concentrate reject 
10%: Feldspar                                     D: Fluting board 
30%: Calcinated kaolin                      E: Bleached pine sulphate
                                                            F: Bleached birch sulphate 
                                                            G: Shredded paper 
                                                            H: Base clay body Ml 


Table 1 Ultimate bending strength of unfired and fired test bars N/mm2
              with varied fibre contents. 
                                                        unfired.                     fired
fibre content                              2%   10%   20%        2%    10%  20%
Pulp reject                                   1.8    1.4     0.8          19.4  7.0   2.5 
Ground w                                     1.8     1.5    1.0          16.8  5.9   2.2 
Recovery concentrate reject     1.2     1.9     1.0         15.4   7.5  1.8 
Fluting board                               1.4     2.1     1.2         19.3   9.0  1.7 
Pine sulphate                               1.8    1.5     1.0          17.8  7.6   3.2 
Birch sulphate                             1.6    1.5     0.9         18.4  10.0  3.2 
Shredded paper                          1.8    1.6     2.6         20.6   5.3   6.8 
Base claybody Ml                        1.0                          24.8 

Table 2 Bending flexure of unfired test bars in millimetres with varied fibre contents.
                                                             2% 10% 20% 
Pulp reject                                           0.8  0.6  0.7
Ground wo                                          0.7  0.8   1.1 
Recovery concentrate reject            0.5  0.9  1.0 
Fluting board                                      0.4  0.9 0.8 
Pine sulphate                                      0.8  0.8  1.0 
Birch sulphate                                    0.5  0.8  1.4 
Shredded paper                                 0.8  0.8  1.4 
Base clay body MI                              0.2 

The bending flexure of unfired paperclay batches with resistance to breaking strain increases manyfold with all fibre types in comparison to that of pure clay. Even with a small ratio of 2% fibre, up to a fourfold bending flexure can be reached. A greater fibre content (10 or 20%) will improve bending flexure with most fibre types but the effect is not any longer remarkable with all fibre types. The best effect was achieved by adding 20 % shredded paper. Also the green strength is improved by adding fibre almost to a double to that of pure clay. However, a fibre ratio exceeding 2% did not generally improve, but reduced the strength rating. The best result was obtained by adding 20% waste paper.

The strength of the fired test bars reduced with all fibre types in comparison to pure base claybody. An increase in the fibre content further reduced the strength rating. Ultimate bending strength fell down by 17% with a 2-percent ratio of the fibre with the best rating at each point. With a ratio of 10% fibre, the decrease was 60% and with 20% it amounted to 73%. After pure base claybody, the best result was reached with a ratio of 2 % waste paper.

The strength and bending flexure rates were affected by how evenly the paper fibre was mixed into the clay, which varied among the test bars. Since the defects were related to production technology, making the bars differ from each other, the defects influenced rather the general strength level than that between the test pieces.

Since the aim of adding fibre was to improve the plastic properties of the clay in green state, the bending flexure was of great importance. A large, thin piece must endure its own weight at all stages of production. The more the clay withstands warping in green state, the larger and thinner the piece can be.

The test results show that a crucially better bending strength and tensile strength was yielded with all fibre types than that of pure base claybodies. However, the fired strength in turn decreased. It can be concluded from the results that the fibre type does not have as essential an effect as the ratio of adding it to clay, when aiming at the best possible green strength without reducing the fired strength. Moreover, the properties of the base claybody and vitrification degree have a crucial effect.

3. POROSITY AND SHRINGAGE

Waste papers often contain traces of clay and filling agents, mainly kaolin. For this reason, it must be taken into account that inorganic substances may affect the vitrification of the voids left by the fibres. A large content of paper fibre can raise the vitrification temperature of the clay in case it contains lots of trace minerals (3). The higher the ratio of paper fibre, the lighter and more fragile is the fired result.

As the clay is vitrified, the piece shrinks and the voids left by the fibres are filled up. Consequently, the vitrification level will essentially affect the-fired strength. The impact of the fibre type to the porosity and shrinkage was studied. The test batches were prepared in the same manner as with the bending fatigue tests. From slabs pressed on plaster bats test pieces with the dimensions 40mrn x 70mm x 5mm were cut and fired to 1100-1250oC in a gradient kiln where seven different temperatures with an interval of 25oC can be used during the same firing. The porosity and shrinkage rates were studied through measuring the water absorption of the test bars. 

Pieces were made from different paperclays. The practical experiments showed that when earthenware is mixed with paper pulp, the strength of thin objects is not sufficient in the temperature of 1100oC. The claybody used in bending fatigue tests was therefore replaced base claybody no 3 containing ball clay and talc and having a lower firing temperature (1100oC). Moreover, the ratio of fibre was changed to correspond the strength level required by green strength and fired strength. The claybodies were mixed with 7% of dry fibre in relation to dry weight of the claybody.


Table 3 Porosity and shrinkage with base clay body 3 and a paper fibre ratio of 7% in seven temperatures. 

                                                                                      A              B               G                H                 I                J                K                 L
Water absorption 
1100oC                                                                        28              32              26             16               27               16              29              31 
1125oC                                                                        26              28              21              13               24                5              25              25 
1150oC                                                                         21              23              14               8               18               47              21              21 
1175oC                                                                         13              16                7               1                                    9              12               14 
1200oC                                                                        10              11                 1               0                 4                                 11               11 
1225oC                                                                          9               9                1               1                  2                                  9                9 
1250oC                                                                          8               9                1               0                 2                                 10               8 
Shrinkage (drying)
                                                                                      0               0                3              1                  1                0                1                 3 
Shrinkage (firing) 
1100oC                                                                          3               1                 4              4                  4                7                3                4 
1125oC                                                                           4              3                6               6                  4                9               4                6 
1150oC                                                                          7               6                9              9                   7                7               6                7 
1175oC                                                                          9               9               11              13                 10               9                                10 
1200oC                                                                        10             10               14             13                 11                11                                13 
1225oC                                                                         11             10               13             14                 13               10                                11 
1250oC                                                                        10             11                14             14                 13               11                                10 

Composition of clay body and paper fibre types
Base clay body 3                                     Fibre types
Ball clay Hyplas 71   45%                     A   Pulp reject 
Talc                            35%                     B   Ground wood pulp reject 
Calcinated kaolin      20%                    G   Shredded paper 
                                                              H   Base clay body 3 with no fibre 
                                                               I    De-inking reject 
                                                               J   Cotton linters 
                                                               K   Fluffed sulphate pulp 
                                                               L   Liner board                                  

4. BENDING IN FIRING 

The firing temperature is significant to fired strength of a thin paperclay object. The clay should be vitrified as thoroughly as possible without warping the object. Once the voids left by the fibres start to fill up in firing, the form is easily warped. With the same test batches of which the porosity and shrinkage was measured also the bending in firing was measured with three different temperatures: 1120oC, 1160oC, and 1200oC.

Test pieces with the dimensions 40mm x 5mm x 150mm were prepared by cutting from slabs pressed on plaster bats. The test bars were placed on a V-shaped base, supported at both ends. After firing each bar, placed horizontally had bent and also shrunk in the middle. Each bar was then removed from the base and its exact profile was drawn on scale paper, from which the bending in firing was calculated.

Table 4 Bending in firing with base claybody No 3 and a fibre ratio of 7%
Bending in firing (mm)             A           B           G             H             I          J           K           L
1120 oC                                       2           6            9             5             3          6           6           6
1160 oC                                      13           9           19            5             9          11          10         10
1200 oC                                     18         20           23           13           18         15          19         18 


5. PRACTICAL APPLICATIONS 

Beside the material study, objects were produced to test the properties of paperclay in practical applications. This way information was gained on the plastic properties of the paperclay in green state as well as its capability to maintain the form intact in firing. Since sufficient green strength can be achieved with all paperclays, the selection of fibre type is affected more remarkably by how easily [available the fibre can be found].
      
[The b]ending flexure of unfired objects is slightly stronger but the fibres tend to build [more] easily while pressing to a plaster cast. Additionally, the thickness of the fibre [in the clay]affects the plastic properties of the clay. Fibres from hardwood having long [fibres] with thick walls are intertwined and build clots more easily in moulding stage [compared to] fibres from softwood that are short and thin-walled. These short fibres are [covered] with small fluffs to which the clay particles will be affixed well.

On the basis of the knowledge gained from the material study, several modelling [methods] were tested. The aim was to find suitable working methods for large thin [objects]. Paperclay can be hand built as well as cast or pressed into mould. However [it differs] from conventional claybodies: while being more un-plastic, it remains plastic [longer] in plastic state, making it difficult to have large three-dimensioned forms [maintain] their shape without a supplementary mould. To make compact objects from [this] material, a mould is required. 

Paperclay has a tendency to attach to a plaster mould tightly, especially with claybodies having a high fibre ratio. Talc was applied to the surface of the mould to make it easier to remove the object from the mould. As the drying shrinkage is smaller with paperclay than with conventional claybodies, the clay should contain enough water and plastic substances to make the object shrink sufficiently to be taken out of the mould. Otherwise removing thin objects without breaking is problematic.

  

With paperclay, it is easy to produce large surfaces, which makes it a natural material for paintings made with the methods of ceramics.  The objects were painted with ceramic pigments in green state. A smooth surface was attained with a plaster mould on which the paintbrush ran smoothly. By pressing to mould, large thin objects bowls and slabs were made. The light weight of the paperclay allows a rolling shape where the round bottom touches the ground only at a couple of points. The bowls were fired on the thin brim whereby the arched structure was able to bear the weight of the material. The slabs were pressed on a plaster bat and their surface rolled tight.Working with paperclay is more spontaneous than with conventional clay bodies. Paperclay lasts quick drying and firing without cracking or warping. The objects were left to dry overnight on the top of the kiln without a plastic cover and taken out of the moulds in the morning when dry. Thin paperclay objects need not be bisqued. Instead, they can be fired directly to the final temperature, since they are very fragile as the fibres have burned away, but the clay has not yet vitrified. However, paperclay endures bisque firing, too. Unfired paperclay absorbs glazing to itself, making the surface compact and improving the fired strength at the same time. Raw glazing also softens the colour surfaces. Thanks to the reinforcing fibre, a glazed, wet object can be safely transported to the kiln and firing can be started when the object is still wet.

Paperclay questions traditional restrictions prevailing in ceramics. Use of fibre improves the plastic properties of the claybody, and gives more creative freedom, from which novel ideas can be brought about. One of the practical applications of paperclay could be e.g. elements for interior decoration where fire-resistant light materials are required.


REFERENCES
(1) Zani, Tengalia, Panigada, Re-use of Paper-making sludge in brick production, ZI 1211990 
(2) Rosette Gault, Paperclay for ceramic sculptors, Studio companion, Seattle WA, USA, 1993 
(3) Rosette Gault, Second-generation ceramic sculpture technical and aesthetic potential of paperclay, Interaction in Ceramics, UIAH, Helsinki, 1993 

Paper presented and published at the 8th CIMTEC World Ceramics Congress, Finenze, Italy June 1997. 
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Reproduced here with kind permission from  Leena Juvonen, Päiväperhontie 12, 00730, Helsinki, Finland

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