Slip to Plastic Clay - De-Watering with Vacuum Bag

The following video clips illustrate my vacuum bag process for removing excess water from my slip mixed clay body tests. This is a reasonably simple approach to achieve maximum plasticity for a clay body test, without the need for maturing the clay over a long period of time.  Using this method a small shop clay worker can easily go from wet slip to fully plastic clay in a single week. Hand mixing clay tests without the slip wetting step will not produce a body test with anywhere near its full plastic potential. If clay tests are not fully wetted the results can be so misleading as to clay working properties and be a total waste of time to prepare.

Part 1: Mixing clay slip for vacuum de-watering

The inspiration for my method came directly from The Potter's Alternative by Harry Davis who as far as I am concerned was a mechanical genius potter. The vacuum de-watering solution described in this book was limited by then available tools and materials and so perhaps anachronistic today. Nevertheless—his basic assumptions were innovative and sound and I offer my enthusiastic recommendation for his now out of print book which is occasionally available used on Amazon. I also heavily referenced Harry's book when I designed and built my double auger pug mill which I will describe in future posts.

 Part 2: Removing excess water from the slip clay.

 My vacuum water removal method does require a few exotic bits & pieces but the overall cost is not prohibitive for anyone who seriously wants to formulate & test mix their own clay body recipes. My vacuum bag was supplied by Vacuum Bagging Systems in Brunswick, ME. The owner Darryl Keil has clay working experience to support his recommendations for the required parts (including small vacuum pumps). Darryl's business supplies vacuum bagging equipment to custom wood workers as the preferred method for clamping 3 dimensionally curved veneers.

Any vacuum bag supplier servicing the wood working industry can supply the required bags and parts. The single open ended filter bag used in my system was supplied by Crosible—the same company that supplies filter cloths for my Netzsch filter press.  My system draws its vacuum from my pug mill vacuum pump eliminating the need for a separate dedicated pump.

Part 3: Vacuum bag de-watering to produce plastic clay

Turning slipped clay into plastic clay has long been preferred method. This approach removes excess mixing water quickly but does add the expense of purchasing and configuring the required parts.  I certainly believe the traditional approach is simpler and has the added benefit of exercising one's patience (well worth developing).

Mr Issac Button, the last great English country potter, used traditional methods in his pottery—including de-watering slip to make plastic clay. My post illustrating the virtue of Mr Button's time tested method, is inspirational and a must see.

Mr Issac Button - Traditional English Potter

In my previous post, I described the use of a contemporary method of removing water from slip to make plastic clay. To compare my methods with a traditional process—I found on Youtube English slipware potter Issac Button, whose mastery of the craft is astonishing. Comparing his method to mine is thought provoking with regard to my much more complicated solutions. Perhaps Wordsworth's wisdom illuminates the relative poverty of our approach—"Getting and spending we lay waste our powers." However you parse it—my small skills evaporate in comparison with Mr Button.

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Bentone EW - Glaze Suspender

In the process of trouble shooting recent crystalline glaze spraying problems I found and carefully read posts by John Tilton regarding his experience using Bentone EW. Concluding it must be finer 'n frog's hair I thought I would give a try—until I found none of our suppliers stocked or had even heard of it.

A prolonged Google search revealed Elementis as the source of Bentone EW which in New England is distributed by TMC Materials INC. At $6.52 per lb I briefly thought my supply problem solved until I learned of their 50 lb minimum. Since they were not able to suggest a small quantity supplier I briefly considered selling small lots of the 50 lb bag on our eBay store. Fortunately my continued search identified Dallas based Trinity Ceramic Supply as stocking and selling small quantities of this elusive powder. A quick call confirmed a 1 lb order @ $10.50 + shipping. As far as I can determine (my determination is definitely fallable) Trinity is the only domestic company supplying small quantities of Bentone EW?

Bentone EW — Supreme Suspender ?

For the technically inclined— DigitalFire publishes the following analysis as well as a detailed description of the product which is probably not derived from actual use.

C.E.C.: 78 me/100 gm
Principle Exchange Ion: Na
pH (5% solids slurry): 8.5
Hectorite Content: 90%
Minus 50 mesh: 90%
Minus 325 mesh: 99.5%
Surface Area: 600 sq. meter/gram
Bulk Density (not compacted): 0.24-0.37 g/cc

Perhaps this supplier information will assist those wishing to purchase and test the use of Bentone EW.

Sink Making Process: Series Three

                                 Part 7: Artisan Style Bath Sink Construction

Hand formed bath sink overflow addition  is demonstrated.

 

                                                        

                                       Part 8: with hand Formed overflow

The hand forming of a bathroom sink overflow tunnel is demonstrated.

                                                                         

Part 9: Hand Formed Overflow On Porcelain Sink

The process of hand forming an overflow on a 18 inch OD rimmed style sink which will shrink to 16 inch OD after glaze firing.

Sink Making Process: Series Two

                                     Part 4: with style changes and reduced size

Traditional wheel throwing techniques are used to reduce the diameter and change the rim style of a pressed sink form.

                               

                   Part 5: Small Vessel Sink Styled and Sized with Custom Detail

Ram pressed porcelain basin is hand formed and altered to re-size as a small bathroom vessel.

                                                                   

                              Part 6: Pressing and Drain Overflow Addition

Bath sink is hand altered on foam forms with traditional wheel techniques to re-size and re-style the custom basin.

                                                                      

                          

Sink Making Process: Series One

This series of videos provide a detailed description of each step in the process of making my Water Stone line of bath sinks at Maine Kiln Works. The videos provide a detailed insight into the tools and processes, both traditional and contemporary, which are used to create the Artisan styled sinks.  In 2001. After 30 years of hand throwing bath sinks—I added a 60 ton Ram Press and began press forming sink blanks which are then individually finished by hand. This post-forming alteration process allows 3 sink styles and 3 sink sizes to be finished from a single resin die made to press an 18 inch OD overflow sink blank.

                                  
                                   Part 1: Ram Press Formed Sink Process

Maine Kiln Works uses the Ram Press Process in an unusual way which leverages both modern industrial and traditional clay forming techniques. The press produces sink cross sections which are much denser than hand thrown sinks—reduces drying & firing distortion, limits shrinkage while producing a more durable high quality bath appliance.

                                                 

                                           Part 2: Hand Detailed Bath Sink

Press forming and hand finishing demonstration & description—three different styles and sizes of custom detailed bathroom sinks.

                                                                          

                             Part 3: Rimmed Porcelain with Custom Detail

Contemporary bathroom vessel style sink—altered with hand detailing and custom sizing.

                                                                      

                              

Glaze Thickness — Dial Indicator

For a number of clay workers the (drop style) dial indicator has proven adaptable for gauging glaze thickness. Machinists' use this instrument to measure small linear distances. As it relates to glaze application—the dial indicator can be adapted to gauge glaze thickness on a bisque pot. John Tilton provides an explanation of his indicator use on his blog.

Dial Indicator — Glaze Thickness Gauge

The precision of dial indicators offer potential advantages but also certain limitations. Because the instrument measures very fine linear dimensions, the margin of error (due to operator application of the tool) can be relatively high when gauging thin glaze thickness and the margin of reading error relatively less high when measuring thicker glaze thicknesses. What am I talking about here?

Visualize the undulating (not flat) vertical surface of a mug. If the indicator foot is placed on top of a throwing ridge and then placed over a valley between ridges you will get two different readings of the same glaze thickness. The indicator must be placed very carefully tangent to the glaze surface being measured and additionally the instrument foot must be placed so you are penetrating glaze thickness only and not air + glaze thickness. If you place the indicator over a valley between throwing ridges you will end up measuring the height between the top and bottom of the throwing ridges + (as you penetrate the raw glaze surface) the thickness of the glaze. If your glaze thickness is .1 mm and the throwing ridge is .3 mm high then you will get a significantly useless glaze thickness measurement on this thin glaze thickness example. On the other hand—if you are measuring a thicker 3 mm crystalline glaze your measurement would be in error by the relatively smaller .3 mm difference between throwing ridge peak and valley and so your measurement of the thicker crystalline glaze would be relatively speaking more useful (though still inaccurate).

Using this machinist instrument will be challenging for many potters because most are not accustomed to measuring these relatively tiny distances and their inexperienced result could easily be incorrect glaze thickness measurements and / or confusion. Despite the above mentioned challenges—my recent oxidation iron red tests and my crystalline glaze trials are two reasons I have begun exploring dial indicator use to measure and record glaze thickness.

Some time ago—I purchased (on eBay) several used Starrett indicators averaging $10 each. The sensitivity and range of each indicator was somewhat different and allowed us to compare inch and mm scales. Picking the model best adapted to glaze measuring can be confusing because there are hundreds of variations offered. John Tilton uses the Starrett Model JZ 643 which comes with a height adjustable hardened steel foot whose dimension and form he altered to meet his requirements. Bill Boyd has also mentioned his use of this instrument for measuring glaze thickness. Picking an instrument with too fine a gradation will prove difficult to manage and picking one with too coarse gradation will sacrifice precision.

Glaze Thickness Indicator — Parts disassembled

The eBay purchase I found most easily adapted is a Starrett # 644-881 (shown above & below) with .01 mm gradation and 5 mm total measuring range. This is .01 mm divisions x 100 per revolution = 1 mm. This scale and range is ideal—so any indicator with similar specs will serve for glaze thickness use. Each revolution of the needle around the dial measures 1 mm making it easy to visually track the thickness measured. For comparison—equivalent inch scale would be approximately .0004 inch. 3 revolutions on a .0004 inch scale would be .120 inch (a bit less than 1/8 inch). My mind finds the millimeter scale easier to follow.

Indicators are either spring loaded so the plunger is retracted at rest or alternately spring loaded so the plunger is fully extended at rest. This Starrett # 644-881 is a fully extended model which is not ideal but was purchased because the cost was $12.00 delivered. Yes—you will need to patiently troll eBay to find what you want for a reasonable price. Similar new indicators typically sell on eBay for for $80.00 > $100. New from a Starrett supplier will range from $175 > $300 depending on configuration and dial size. Reversing the spring loaded rest position is easily accomplished by unscrewing the top plunger button and then placing a small spring beneath and re-attaching the button. Various removable pins (with standard thread diameters and pitches) are inexpensive and readily available through machinist supply firms such as Enco. Or alternately—whatever pin comes with your used indicator can be chucked in a variable speed drill and reduced to a small but sturdy needle diameter by placing the drill driven pin against a power grinder, belt sander or 8.5 x 11 abrasive sheet. The pin should be very slightly rounded at the point but small enough diameter to readily penetrate the raw glazed surface.

An excellent online source of dial indicator information (as well as sales and service) is Long Island Indicator. Their site is full of useful information and a good place to start if you want to understand this instrument in greater detail. This Starrett PDF provides general information regarding various model and specifications.

After obtaining a suitable indicator you will need to fabricate a foot appropriate to glaze thickness measurement. Though Starrett sells hardened steel indicator foot bases—it is much simpler and less expensive to make a foot from a stable plastic such as Acetal (Delrin) which can be easily obtained in small quantity from Online Metals or eBay small lot sellers.

Dial Indicator with Delrin foot removed to show glaze penetration pin

A foot long length of Delrin can be purchased for a few dollars and will provide stock to fabricate several indicator feet as needed. Cut a short length 1.5 > 1.375 inches long and then center bore a 3/8 inch diameter hole 3/4 of the length of the Delrin round section. Then drill a 3/16 inch hole from the opposite end so that it bores into the previously drilled 3/8 hole. The boring is best done on a lathe (either metal or wood) by chucking the Delrin round in the head stock and advancing a drill from the tail stock into the Delrin round. The boring is not difficult and can be accomplished by carefully using a drill press or hand head drill. You simply want the holes to be reasonably concentric.

Starrett indicators have a 3/8 inch OD plunger barrels (as shown above) which will create a tight friction fit on the 3/8 inch hole drilled in Delrin round stock. You will have a tight friction fit because drilling the hard but very slightly flexible Delrin plastic will cause this 3/8 inch hole to be very slightly undersized. This friction fit will allow you to easily twist adjust the height of the Delrin foot so that the resting pin height and Delrin foot are zeroed in the same plane. All of this is much easier to see with your finished instrument than to verbally describe.

Unless you are of a precision bent—you will probably do well to ignore this anal-retentive post. The more patient among you may be amused by the prospect of investing $30 > $40 (including misc parts and shipping) + careful fabrication time to obtain a curious and perhaps useful impediment to your creative work.

Iron Red Test Firing - 3d Series

My 2nd iron red test firing indicated a 3rd series of tests would help me better understand the variables.  My 3rd test series will explore glaze thickness and firing schedule in greater depth.

Glaze thickness has proven a sensitive variable creating substantial variations in color and tonal range. For this Bailey's Red series I created another line blend with Spanish Red additions from 8% to 14%.  I also carefully recorded the thickness of the glaze on each test using a shop-built dial indicator glaze thickness tester. This clever device is quite similar to the tool John Tilton is using to measure his crystalline glazes. I will discuss my adaptation of a dial indicator for glaze measuring chores in the next post. I will include complete information for anyone who would like to know more about my process for adapting an Ebay purchased dial indicator to measure glaze thickness.

The below test series follow suggestions by John Post to test cooling ramps from 150F /H to 125F /H  through the 1800F  > 1300F range.  Below are the results of this slower ramp with a 8% to 14% iron oxide line blend. All are fired @ cone 10 oxidation over our standard T68 stoneware.

Bailey's Red 8% > 14%—cooled at 125F /H

The above had only very slight variations in red toned results from our earlier test tiles which were cooled at 150F /H. The results were not significant enough to conclude that slowing the cooling rate improved the vibrancy of the iron red color. My most favored test (Bailey's Red 12%) had similar bright red color whether cooled at 150F /H or 125F /H.

Thickness, however, did prove to be an important factor to maximize the brilliant iron red color. I tested Bailey's Red 12% with two different glaze thickness. The below examples show a thickness of .5mm on the left and .6mm on the right, accurately measured with our dial indicator.  With a minimal increase of .1mm, the formation of the olive / black toning appears to have a significant effect. The effect of this slight difference in thickness is certainly surprising and I will do follow-up to be confirm this conclusion.
 

Bailey's Red 12% with .4mm and .5mm glaze thickness

Being able to use the dial indicator to record glaze thickness has proved to be interesting and useful. However I would not consider this tool a necessity for producing iron red glazes. Certainly potters have (for a great many years) carefully controlled their glaze thickness application with much simpler and less expensive methods. A simple needle tool can be quite effective in this respect.

The below mugs (from the same firing) illustrate varying thickness of 12% Bailey's Red from .4mm, .5mm and .6mm.  The left mug (with thinnest application) shows thin spots of dark metallic tone. The center mug produced iron red which is close to my best results and the third mug alternated the olive / black tone speckling with mingled patches of red tone.

Bailey's Red 12% with .4mm, .5mm and .6mm glaze thickness

So application of glaze does appear to be an important factor for iron red development—as long as these results are understood as a result of my test schedule, kiln equipment and glaze recipe. In other words your results may not match mine. For anyone who interested—I will give a careful explanation of purchasing, modifying and using our glaze thickness / dial indicator in the next post.

I am also going to do follow-up testing @ 100 F /H from 1800F > 1300F.  Since this is a substantial difference in cooling from the first firing (150F) and this current firing (125F)—the results could be curious and interesting.

Iron Red Test Firings — 2nd Series

The first iron red test fire in my Baby Doll kiln produced worthwhile & interesting results. The size of this kiln is quite deceiving; I assumed at first glance that the kiln was not worth the time nor effort (or cost) to fire. I was indeed wrong and the results gave me a strong incentive to follow up with additional tests.

L & L Doll Test Kiln

The circular shelves that L & L sells were a bit thick for so small a kiln so I purchased thinner high alumina shelving from Bailey Ceramics Supply and then cut the rectangular shelves to a hexagonal cardboard template that would maximize the load space for this tiny kiln. The shelves are sized to allow approximately 1/2 inch circulation space between the shelf and the kiln walls. One shelf has the front edge cut back so the cone can be placed in front of the spy hole without striking the front shelf edge.

Shelves and Posts

Cutting high alumina shelves is not too difficult, assuming one has the appropriate tools. The ferocious water cooled cutting beast (shown below) with diamond edged blade and 1.5 hp motor (that I salvaged and rejuvenated some years ago) allowed the cutting process to be quickly accomplished.  Small left over scraps were trimmed into tiny shelf posts, using the same machine. I measured the height of my tests tiles, so the cut posts would maximize the kiln load.

Diamond Cutting Saw

For the second test firing, I decided to concentrate on the Bailey's Red with 12% red iron oxide (Spanish Red was used), hoping I could achieve the same vibrancy and stability when applied to functional ware created at Maine Kiln Works.  I have been using GlazeMaster to record and update my tests. It is a resourceful program for creating line blends, developing glazes, and creating batch sheets for glaze tests. Created by John Hesselberth of Frog Pond Pottery, it is a glaze calculation program (designed for studio potters) which can also be used for clay body recipes.

For this particular test firing, I created a larger batch of Baileys Red 12% red iron oxide. Using GlazeMaster, I expanded the glaze batch to 5000 grams from the 100% base recipe.  Below is the glaze recipe published in John Britt's book. This is base glaze I am testing with 12% iron oxide substituted for the original 8%.

Kaolin - EPK 13
Felspar - Custer 47
Talc - 10.5
Bone Ash - 14
Silica - 13
Lithium Carbonate - 2.5

Iron Red Oxide -12

I tested the glaze line blend over stoneware and porcelain mugs and flat dishes in order to get a contrast between horizontal and vertical glaze effects with the same glaze thickness. All the pieces were applied been dipped for timed interval into the 5000 gram batch.

I kept the firing schedule the same, repeating John Post's recommended 150 F / H slow cooling ramp from 1800F to 1300F.  These are some of the results from this firing.

Stoneware Dish

With slight variations, the 12% iron oxide Bailey's Red test revealed the above result on this stoneware chowder bowl.  It did not however, create a similar effect on the porcelain dish. The formation of iron red crystals on the surface did not form to a great extent, and left the porcelain with predominantly brown > black tonal range with very little red crystal development.

Porcelain Dish

Likewise the mug results showed the same browner tone over the porcelain example as shown in the centre, with the left and right mugs showing the slow cooled iron red over my shop developed T68 stoneware.

Bailey's Red 12% over Stoneware (L,R) and Porcelain (C)

The iron in the stoneware body must play some role in the formation of iron crystals on the surface of the glaze. As there is very little to no iron content in the porcelain body, then the results achieved are different.  If anyone is working in a similar process and their results are different on the porcelain, please leave me feedback, it would be greatly appreciated! It would interesting to achieve a good iron red over porcelain as well.

I recognized a slight difference in color and crystal formation from the previous firing, noting that Bailey's Red did not turn out as bright as the previous firing results. I have taken into consideration  glaze application and thickness are crucial in achieving a bright iron red.  I will fire the kiln again, and consider modifications to glaze thickness as well as the cooling ramp. My initial thoughts after this firing is to make another line blend but vary the thickness of the glaze. I am also going to take up John Post's recommendation of adjusting the cooling temperature from 150F per hour to 125F per hour from 1800F to 1300F. My next post will display these results.

My generous thanks to John Post for his explicit and very helpful suggestion which have definitely put me on the right track with the beautiful and somewhat unusual oxidation glaze effect.

Only a bit unrelated to the current subject—I will be posting blogs with information which will detail a very effective process for cutting the very hard St Gobains Advancer (distributed by kilnshelf.com) kiln shelves that are both expensive and widely used due to their strength, longevity and flat firing qualities. One would think their hardness would make them virtually impossible to cut but they are actually fairly easy to cut if one knows what process to employ. The solution is water jet technology which is widely available in many locations. Stay tuned for more information in this regard.