The basis of ceramics is clay but other materials which affect the clay's properties are added to the clay or occur in it naturally.
STONEWARES - are fired at more than 1100 degrees to give a non-porous ware.
EARTHENWARES - are fired at 700 to 900 degrees and are porous.
For Viking Age pottery we are discussing EARTHENWARE - often poorly fired.
CLAY is principally made up of a specific group of minerals which originate as other minerals and are later chemically degraded at the earth's surface. The clay mineral particles so produced are then often transported by water or ice or wind to other places where they accumulate in layers pure enough to be worth using for ceramics. Many soils include some clay but unless the clays are the main component of a deposit the material is not PLASTIC enough to be worked.
PLASTICITY is the property which makes clays so useful. A PLASTIC material can be deformed and then maintains the deformation when left to rest. You can deform water but it constantly shifts form according to gravity because it is a FLUID. A brick cannot be deformed unless by breaking it because it is fired and has lost its plastic properties. Clay can be formed as desired and then fixed in that shape by firing because in its unfired state it is PLASTIC and because FIRING changes its physical properties as heat affects the minerals present.
HAVE I FOUND A CLAY ? Clay particles are mostly so small they cannot be individually seen with the naked eye so they are identified with some simple tests.
DEFORMATION : Poke the deposit after wetting it. If it does not crumble or break and retains the marks then it is plastic. Take a small piece and squash it in the hand. If it stays deformed without crumbling then you are getting warmer. Now roll a piece the size of a walnut in the hand to make a sausage. If the sausage can be rolled to less than 5mm thick this is a clay. Broken sausages mean the clay is too dry - try again after wetting - or it contains too much non-plastic material.
PURITY : Smear the clay out in your hand and see if sand, gravel or humic matter is included. Dry some out and crumble it to see how much of this is present. Too much of this stuff will make working with the clay tiresome. It can be removed.
The best thing to do now is to take a lump and make a TEST FIRING. A test firing should be a small tile circa 1cm thick and of side 10cm. Fire to 775 centigrade, then 900 then 1050. Be warned !-- firing too high may cause a meltdown in your kiln.
Before you dry the tile mark a 10cm line on it. Measure this after firing to see the percentage of shrinkage. Shrinkage over 10 percent may causee problems for potting - items may crack and distort.
N:B: At stoneware temperatures clays can start to expand rather than shrink !
CLEANING AND PREPARING CLAY should be done dry - IMHO. Wet processing requires tanks of mud, time for settling and drying and is just not necessary.
First, you should slice or crumble your wet clay onto cloth or plastic sheets in a place where it can dry out. A windy place is best. Turn the material each morning and evening.
Second, you should pound and grind the dried material to dust using a stone or hammer on a brick or stone. This creates dust so do not do it indoors.
Third, sieve the dust. Use a fine sieve like those for flour or a frying pan spatter guard. You could also buy a 100 mesh sieve. If you use a sieve which is too fine you will use too much time.
Re-crush large pieces and sieve again. Throw stones etc away.
Fourth, if you will add grog, sand, etc do it now in the correct proportions. Mix.
Fifth, add water, half a cup at a time into a well in the pile of powder and mix like kneading dough for bread.
DO NOT ADD WATER TOO FAST - BUT YOU CAN DRY THE MIX OUT IF YOU DO
The result should be a nice even clay which is damp and plastic but not wet and not crumbly.
Wedge the clay and wrap it for storage.
Look HERE to find out about clay compositions in the Viking Age
STONEWARES - are fired at more than 1100 degrees to give a non-porous ware.
EARTHENWARES - are fired at 700 to 900 degrees and are porous.
For Viking Age pottery we are discussing EARTHENWARE - often poorly fired.
CLAY is principally made up of a specific group of minerals which originate as other minerals and are later chemically degraded at the earth's surface. The clay mineral particles so produced are then often transported by water or ice or wind to other places where they accumulate in layers pure enough to be worth using for ceramics. Many soils include some clay but unless the clays are the main component of a deposit the material is not PLASTIC enough to be worked.
PLASTICITY is the property which makes clays so useful. A PLASTIC material can be deformed and then maintains the deformation when left to rest. You can deform water but it constantly shifts form according to gravity because it is a FLUID. A brick cannot be deformed unless by breaking it because it is fired and has lost its plastic properties. Clay can be formed as desired and then fixed in that shape by firing because in its unfired state it is PLASTIC and because FIRING changes its physical properties as heat affects the minerals present.
HAVE I FOUND A CLAY ? Clay particles are mostly so small they cannot be individually seen with the naked eye so they are identified with some simple tests.
DEFORMATION : Poke the deposit after wetting it. If it does not crumble or break and retains the marks then it is plastic. Take a small piece and squash it in the hand. If it stays deformed without crumbling then you are getting warmer. Now roll a piece the size of a walnut in the hand to make a sausage. If the sausage can be rolled to less than 5mm thick this is a clay. Broken sausages mean the clay is too dry - try again after wetting - or it contains too much non-plastic material.
PURITY : Smear the clay out in your hand and see if sand, gravel or humic matter is included. Dry some out and crumble it to see how much of this is present. Too much of this stuff will make working with the clay tiresome. It can be removed.
The best thing to do now is to take a lump and make a TEST FIRING. A test firing should be a small tile circa 1cm thick and of side 10cm. Fire to 775 centigrade, then 900 then 1050. Be warned !-- firing too high may cause a meltdown in your kiln.
Before you dry the tile mark a 10cm line on it. Measure this after firing to see the percentage of shrinkage. Shrinkage over 10 percent may causee problems for potting - items may crack and distort.
N:B: At stoneware temperatures clays can start to expand rather than shrink !
CLEANING AND PREPARING CLAY should be done dry - IMHO. Wet processing requires tanks of mud, time for settling and drying and is just not necessary.
First, you should slice or crumble your wet clay onto cloth or plastic sheets in a place where it can dry out. A windy place is best. Turn the material each morning and evening.
Second, you should pound and grind the dried material to dust using a stone or hammer on a brick or stone. This creates dust so do not do it indoors.
Third, sieve the dust. Use a fine sieve like those for flour or a frying pan spatter guard. You could also buy a 100 mesh sieve. If you use a sieve which is too fine you will use too much time.
Re-crush large pieces and sieve again. Throw stones etc away.
Fourth, if you will add grog, sand, etc do it now in the correct proportions. Mix.
Fifth, add water, half a cup at a time into a well in the pile of powder and mix like kneading dough for bread.
DO NOT ADD WATER TOO FAST - BUT YOU CAN DRY THE MIX OUT IF YOU DO
The result should be a nice even clay which is damp and plastic but not wet and not crumbly.
Wedge the clay and wrap it for storage.
Look HERE to find out about clay compositions in the Viking Age