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.