Slip and Score by John Britt
SHOULD I SLIP AND SCORE? One of the most common instructions in an introductory ceramics class is: “If you are connecting things you must slip and score.” This is a valid recommendation if you want pieces to stick together without cracking off and an excellent and time-tested practice in pottery. But there are also real world practices that go directly against this proposition, like in industry where they just use slip to connect a piece without scoring, or production potters who just put handles on cups without slip or scoring. These methods seem like a much easier way to attach things but they goes against one of the primary rules of ceramics- to slip and score. How can all three of these methods work when they contradict each other? There must be a reason or set of parameters that makes each method work, so I set out to investigate. This all came about because I was contacted by a ceramic engineer who was having problems with cup handles and casserole lid knobs that inexplicably started falling off. In industry, workers slip cast cups and handles and then, after cleaning up the mold marks, the ends of the handle are dipped into a “joining slip” and pressed onto the cup to adhere it. There is no scoring; just the slip is enough to attach the pieces. This way they can connect lots of handles in a day. If they had to score, slip and clean up each handle it would add many hours to the process. For decades, this 1
Slip and Score by John Britt process worked without fail. The engineer had to find out why the handles were now falling off. The cause of the problem turned out to be quite simple. The company started heating the work space so the workers would be more comfortable. That is when all the handles were falling off. But the question was – What was the cause that made it work all these years so that they could figure out how to keep the heat on and still have the process work. The ceramic engineer found the answer in a book entitled “Predictive Process Control of Crowded Particulate Suspensions” by James E. Funk and Dennis R. Dinger. (It is in what the ceramics engineers call the Green Bible.) In the book they experimented with the above “joining slip” in toilet manufacturing and they found that small perpendicular cracks which occurred in joints as they dried were due to the slip paste being ‘high dilatancy and low moisture.” This dilantancy and low moisture caused both cracking and poor bonding. But if a slip is made with the same casting slip and it instead is flocculated with Epson Salts until it is like cake frosting, it provides a good bond and no cracking. This is called a “stick up slip” because it also has the property of being “sticky”. This slip has an increased viscosity and higher moisture content so when it is used to join the cast pieces they are pulling into tighter contact as they dry. Just as a background, the joining slip is made by taking the same deflocculated casting slip used to make the cups and handles and adding dry scraps until it is thickened. This creates a joining slip that has about twice as much clay as the casting slip (about half as much water). It is like a supersaturated solution of slip. I have been using a similar method of making a “joining slip” from my porcelain throwing clay body and then deflocculating it and adding dried trimming scraps until it becomes very thick like cake frosting. I started using this joining slip with porcelain handles because, without it, they always seemed to crack at the joint and if I used a transparent glaze or a celadon you could see the crack. This works well for me but I also score both pieces that I am joining. So we used a similar method although I was both scoring and slipping thrown pieces and pulled handles vs. their cast pieces.
Making up “Joining Slip” with immersion blender.
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Slip and Score by John Britt After reading about the stick up slip, the factory engineer devised an experiment which provide him with the answer and he wanted me test his experiment with my clay and see if I got his same results.
Test rods that have been labeled and stuck together over the coarse of the day.
TEST PROCEDURE: Overview: Extrude a rod of clay, cutting it into pieces which are connected with the stick up slip at regular intervals over the course of a day. Each is then weighted, marked and recorded. The extruded rod dries more and more as is sits out longer in the air, resulting is a moisture gradient that starts out in the high teens and drops to the low teens. When the pieces are completely dry, weigh them and calculate the percent moisture. Then you try to break the joint, recording if it was easy, medium or hard to break. Here is what I did: 1. Extrude enough rods (1/2� diameter x 1� length) to stick together every 30 minutes for 8 hours. 2. Cut the first two pieces and stick together with joining slip, then cut another and join with stick up slip and another with scoring and joining slip, then ID and record the weight. 3. At 30 minutes intervals, stick another two together, ID and weigh. 4. Continue this to the end of the test. Let air dry overnight and then put in a dry box with heat and air. 5. Record the dry weight of each pair and calculate the moisture content of each stuck pair. 3
Slip and Score by John Britt (Wet weight – Dry weight ) Wet weight
x 100% = % Moisture content
6. Now try to break each pair apart and judge the effort and record. My test validated their tests- -if the moisture was around 14.5% it was very hard to break the joint. (See Joining Test Data page 9) For this non-scoring method to be successful, you must carefully monitor the moisture of the pieces. If kept at 14.5% you can join pieces all day and not have handles and knob fall off with only stick up slip. (No scoring). This solved the problem and also showed it was the heating of the factory that caused an otherwise foolproof method to fail. The clay was getting too dry in winter when they started heating the factory to make the workers more comfortable. Weighing rods .
Studio Potters What does all this mean for studio potters? Because we usually slip and score to join pieces -does this mean we should stop? I don’t think so but it does validate that it is the Best Method in the Joining Test Data which shows slip and scoring as most effective. But in a factory, scoring is too time consuming and often leaves marks which can be undesirable and in order to stay in business, time is of the essence. While studio potters often make one-of-a-kind or small runs of items so scoring is added insurance that parts will stick together. Time is not the main consideration. If you risk not scoring and even 10-30 % or your handles fall off that is the loss of thousands of dollars. So scoring is insurance that the handles will stay on. Potters are always managing heat and humidity in our studios and this test shows how important controlling drying is to our success. In winter when the heat is on – it is different than in the summer with the windows open, which is different than working after a week of steady rain or a very hot summer with air conditioning. Because of the change in heating at the factory they had to purchase a $3,000 moisture meter and also use wet boxes to control humidity. Many potters have wet or dry boxes to control the drying of the work. I have been using these for 30 years after reading about it in Bernard Leach’s 4
Slip and Score by John Britt “Potters Book.� I pour 2 inches of plaster in the bottom of a plastic container and let it dry. Then as I use it I just add water as necessary to make it either a wet box or a dry box.
Pouring plaster into large plastic bin to make Wet/Dry box.
There are many methods in ceramics because of the variety of ways things are formed, dried, fired and glazed. In this example the ceramic manufacturing company formed pieces by slipcasting and jiggering while studio potters usually throw or slab build. Also ceramic manufactures dry things faster and fire much faster (they usually single fire in oxidation gas.) When a studio potter throws a cup and the next day trims and adds a handle and then covers the piece for 2-3 more days this is very different and slow way of working can actually equalize moisture when using the joining slip method. So studio potters may never encounter this problem.
Other practices in studio ceramics: Vinegar vs. Magic Water With the above experiment mind, I think a bit more can be said about other methods in studio ceramics that appear at odds. Namely using vinegar (flocculant /acid )to connect handles or various types of deflocculated slips or magic water. These appear to be the opposite principles and yet both groups of potters swear that their methods are the most effective.
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Slip and Score by John Britt 1. Method 1 involves connecting handles using vinegar (mild acid/flocculant) instead of water . You just score and add some vinegar to the score lines and do the same with the handle and then attach. Many potters swear by this method. 2. Method 2 involves connecting handles using Magic Water (a mild base/deflocculant ) instead of vinegar or slip to the score lines and do the same for the handle and then attach. Many potters swear that this is the best method. These two systems both work and yet they are opposite principles. How can this be? They are different than the stick up slip and joining slip method in that you are scoring and working in the liquid to the clay joint. I think that the magic water and vinegar are red herrings in this example because I think the overriding feature of these methods is SCORING not necessarily the vinegar (flocculant) or Magic Water (deflocculant). Another factor is that in studio pottery you don’t dry things too fast. Both methods recommend low drying by covering or waxing joint.
No Scoring or Slip This brings up to another practice; just joining a handle to a cup with no slip, no deflocculant or flocculant and no scoring! This is a common practice with production potters. They just throw the cup shape and as it is drying they make handle blanks and then at the RIGHT TIME, when the cup is slightly stiff so as not to totally deform while handling it, they put on the handle blank and pull it right on the cup! (see photo) The trick here is getting it at the RIGHT TIME and that can take years of practice of just the right clay body because it doesn’t work on all clays. The cup is thrown and allowed to dry probably to about 14-15% moisture and the handle blanks are formed and allowed to dry probably to about 14-15% moisture and then are conducted. The above experiment may inform why this practice works. The moisture of the pieces is important.
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Slip and Score by John Britt
Here are some handy recipes for slips that potters use to connect or fix things: This is a recipe that Nils Lou used to promote saying that he never scored. What his slip mixture reveals is that you can throw the kitchen sink at the problem. He uses soda which has sugar and is sticky. He uses Elmer’s glue which is sticky. He uses Acrylic Gloss Medium which (sticky) makes the mixture thixotropic (a condition of deflocculation) and then just pours in his morning left over beer. And when in doubt – add more vinegar.
Nils Lou Gorilla Slip 1 cup of Kaolin 1 cup of Ball Clay 1 Cup of Dr. Pepper 1/2 cup of Stale Beer (Corona is preferred) 1 Tablespoon of Elmer's White Glue 1 Tablespoon of White Vinegar (for pH balance) 3 Tablespoons gloss acrylic medium, (Liquitex Acrylic Gloss Medium) The latter product (the gloss acrylic medium) makes the mixture thixotropic, so stir well before using. If desired thin with 50/50 white vinegar and water. This slip will stick anything to anything and stores indefinitely in an airtight container.
Magic Water 1 gallon of water 3 tablespoons of liquid sodium silicate 1 1/2 teaspoons of soda ash
Magic Mud In a blender add one cup of Magic Water and then add 1/4 to 1/3 of a cup of a paper napkin, toilet paper, or paper towel. Blend. Then add 3/4 to 2/3 of a cup of bone dry clay you are using that has been powdered. Blend. Add more Magic Water if necessary to wet entire mixture to the consistency you want.
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Slip and Score by John Britt
Bisque Crack Repair (all cones) White Glue 50% Sodium Silicate 50% Add EPK until thick. Pack into crack.
John’s Bisque Repair (all cones) Appropriate amount of Sodium Silicate (1/4 cup) Add Glomax (calcined kaolin) until thick or preferably add your own clay body that had been crushed into a powder and either bisque fired (cone 05) or soft bisque fired (cone 022) to eliminate shrinkage. Pack into crack and fire. This is the mixture Adelaide Robineau used to repair the Scarab Vase in the 1920’s.
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JOINING TEST DATA 2016 CODE
TYPE
EASY
MEDIUM
A
Slip and Score by John Britt
HARD % MOISTURE X
BEST
19.1
B
Floc
X
18.5
C
Defloc
X
18.5
D
Floc
X
18.0
E
Defloc
X
18.0
F
Floc
G
Defloc
H
Score Defloc
I
Floc
J
Defloc
K
Score Defloc
L
Floc
M
Defloc
N
Score Defloc
X
15.8
O
Floc
X
15.0
P
Defloc
Q
Score Defloc
X
15.0
R
Floc
X
14.8
S
Defloc
T
Score Defloc
X
14.8
U
Floc
X
14.5
V
Defloc
W
Score Defloc
X
Floc
X
13.7
Y
Defloc
X
13.7
Z
Score Defloc
.A
Floc
.B
Defloc
.C
Score Defloc
.D
Floc
X
13.1
.E
Defloc
X
13.1
.F
Score Defloc
.G
Floc
X
12.5
.H
Defloc
X
12.5
.I
Score Defloc
X
16.6
X
16.6 X
16.6
X
16.3 X
16.3 X
16.3
X
15.8
X
15.8
X
VERY HARD
15.0
X
VERY HARD
14.8
X
VERY HARD
14.5 X
14.5
X
13.7
X
13.5 X
13.5 X
13.5
X
VERY EASY
13.1
X
12.5 9
VERY EASY
Slip and Score by John Britt
Other articles by John Britt: (PDF format)
“Is it Food-Safe?” “All that Glitters is not Goldstone” “The Quest for the Illusive Leaf Bowl and other Assorted Articles” “I Need a Cone 6 Glaze that Doesn’t Craze”
BOOKS : “The Complete Guide to High Fire Glazes” “The Complete Guide to Mid-Range Glazes”
DVD’s AND VIDEOS: “Understanding Glazes” “Fundamentals of Wheel Throwing” “Beyond the Basics: Tumblers and Yunomis” “Beyond the Basics 2: Kurinuki” “Fundamentals of Glazing”
All are available on my website: johnbrittpottery.com
Copyright © John Britt 2016
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