82 THE JOURNAL OF PROSTHETIC DENTISTRYVOLUME 85 NUMBER 1
T he proper fit and cementation of a cast restoration
onto a prepared tooth is crucial to both short- and long-
term function. Casting fit is a function of the many
variables related to changes in size and shape introduced
by the materials used in the construction of a restora-
tion; these variables can stem from both the intrinsic
properties of the materials and the clinical technique.
Even if all variables were controlled carefully to ensure a
perfect fit, the restoration still could not be seated prop-
erly because of insufficient space for the luting agent.
Proper complete seating of full veneer restorations,
without an excessive luting agent film thickness, is
often difficult to achieve because of cement hydraulic
pressure.1-7Incomplete seating may result in occlusal
interference after cementation, loss of proximal con-
tact, reduced crown retention, discrepancy of
marginal fit, secondary caries, plaque accumulation,
periodontal problems, rapid dissolution of luting
agents, and/or hypersensitivity.3-6,8-11
Internal relief, or venting of the casting, can overcome
the effects of luting agent hydraulic pressure.2-9,12-19
The use of venting requires drilling holes in the casting.
The casting is cemented in place and the vent holes filled.
Although venting is an effective technique to improve
the seating of the restoration, it has several disadvan-
tages: An extra visit is necessary to fill the vent hole; the
materials used to fill the vent hole may cause microleak-
age and/or discoloration; and, in the case of
ceramometal restorations, occlusal venting may weaken
the strength of the porcelain.
Internal relief avoids the limitations of venting.
Internal relief provides a space between the casting and
the abutment to accommodate the luting agent and the
Effect of evaporation and mixing technique on die spacer thickness:
A preliminary study
Jason J. Psillakis, DDS, MS,aMona E. McAlarney, DEngSc,bRobert F. Wright, DDS,cand Javier
School of Dental and Oral Surgery, Columbia University, New York, N.Y.
Statement of problem. Casting relief is required for proper seating of castings to allow for luting agent
thickness. The application of die spacer to the die is the most common method of obtaining casting relief.
Die spacer film thicknesses that are outside the ideal range of 25 to 40 µm can cause clinical problems.
Thickness can be affected by the separation of die spacer constituents, which may not be reconstituted by
mixing, in the bottle and by the evaporation of volatile components while the bottle is open.
Purpose. The purpose of this study was to determine the effects of component evaporation and die
spacer mixing technique on applied die spacer thickness.
Material and methods. Bottles of Gold Tru-fit die spacer were left open for 0, 1, 4, 8, and 24 hours
at 22°C. Spacer solutions were shaken either by hand per the manufacturer’s directions or on a dental
vibrator for 1 minute. One even brush stroke of spacer was applied to clean glass slides. Three die spacer
films were made for each combination of time and mixing technique. Eighteen thickness measurements
per sample at various sites were recorded with profilometer tracings. Statistical differences were deter-
mined with a 2-way ANOVA.
Results. Handshaking provided greater die spacer thickness, which increased with the time that the bot-
tle was open. Vibration provided lower thickness with no statistical increase with time.
Conclusion. Insufficient agitation caused lower film thickness. Excessive evaporation caused higher film
thickness. (J Prosthet Dent 2001;85:82-7.)
This study was supported in part by grant No. NIH R29DE10980.
aAssistant Professor of Clinical Dentistry, Division of Prosthodontics.
bResearch Scientist, Division of Prosthodontics.
cAssociate Professor, of Clinical Dentistry; Director, Division of
dPrivate practice, Jersey City, N.J.
The results of this preliminary study suggest that insufficient agitation during die
spacer mixing may result in insufficient casting relief for proper seating of castings.
Evaporation may have the opposite effect: bottles open for 4 or more hours are subject
to the evaporation of organic liquid and may at least double the applied die spacer
thickness, leading to excessive relief.
PSILLAKIS ET AL THE JOURNAL OF PROSTHETIC DENTISTRY
marginal expression of excess cement.20Methods of
obtaining internal relief include grinding the inside of the
casting, internally carving the wax pattern, etching with
aqua regia, electrochemical milling, cutting the internal
channel, and applying die spacer to the abutment.
The most popular method to achieve adequate
internal relief is die spacing.3,8,9,16Die spacer is a solu-
tion that is painted on the die before the fabrication of
the pattern. The presence of the spacer provides space
for the luting agent. The use of die spacer reduces ele-
vation of the casting restorations,5,7,12,13,15,18,19,21-23
decreases seating time,7improves outflow of excess
cement,20and lowers seating forces.7Conflicting evi-
dence exists on the effect of die spacer presence on
clinical crown retention.7Grinding, internal carving of
the wax pattern, etching with aqua regia, and electro-
chemical milling are no longer recommended methods
because they are inaccurate and inconsistent, and it is
impossible to achieve uniform space for a cementing
The reported ideal die spacer thickness ranges from
25 to 40 µm.4,11,12,17,22-25Die spacer thickness should
be large enough to allow proper seating of the casting
but not so large as to cause excessive cement thick-
ness.26For optimal results, die spacer thickness should
In general, die spacers consist of metal-oxide pow-
ders and adhesives dispersed in an organic liquid such
as a ketone. All components must be properly dis-
persed and the die spacer composition held constant
for optimal clinical effectiveness. With time, particles
tend to settle to the bottom of a standing bottle. If the
bottle is open, the organic matrix liquid tends to evap-
orate because of its high vapor pressure, thereby
changing the die spacer composition. Mixing tech-
nique and the overall time that a bottle is open during
clinical use can affect die spacer thickness and there-
fore clinical success.
Manufacturers recommend shaking the die spacer
bottle vigorously before application to adequately dis-
perse the components. Replacing the cap on the die
spacer bottle immediately after use and discarding
well-used bottles are also clinically prudent.
To ensure a consistent film thickness during stud-
ies, researchers not only vigorously shake the bottle
but also immediately cap the bottle after application
and/or frequently replace the die spacer bottle. These
optimal conditions of mixing and prevention of evap-
oration do not occur clinically, where bottles may
stand open for some time and shaking is often less
The purpose of this preliminary study was to exam-
ine the effect of mixing intensity and evaporation of
die spacer components on die spacer thickness. To this
end, die spacer film thickness was measured after
allowing die spacer components to evaporate by leav-
ing the bottle open at room temperature for various
times up to 24 hours and then mixing, either by hand
(shaking) or by the vibration of the bottle.
MATERIAL AND METHODS
A Gold Tru-fit spacer (George Taub Products,
Jersey City, N.J.) bottle was left open for 0, 1, 4, 8,
and 24 hours at 22°C. After each time interval, the
bottles were closed (only for the mixing process) and
mixed either by hand (shaking) according to the man-
ufacturer’s recommendations or by placing the bottle
on a dental vibrator for 1 minute to disperse the mate-
rial. After mixing, die spacer suspension was applied
to clean glass slides with a single, even brush stroke. A
new brush was used for each application. Samples
were dried in air at 22°C. Three samples of die spacer
suspension on glass slides were made for each combi-
nation of mixing technique and evaporation time
Die coating thickness was determined using a
Tencor Alpha-Step 200 profilometer (KLA-Tencor
Instruments, San Jose, Calif.) with a diamond stylus
tip 12.5 µm in radius (Fig. 1). A profilometer mea-
sures film thickness by tracing a stylus from an
Fig. 1. Schematic drawing of side view of slide coated with die spacer in profilometer.
Profilometer stylus travels across slide from uncoated area to region coated with die spacer.
When in contact with coating, stylus rises and records its elevation change as coating thickness.
THE JOURNAL OF PROSTHETIC DENTISTRY PSILLAKIS ET AL
84 VOLUME 85 NUMBER 1
uncoated region to a coated region. When the stylus
comes in contact with the coating, it rises with the ele-
vation of the film thickness. The change in stylus
elevation due to the film is recorded.
Three profile tracings were made for each sample
(Fig. 2). For each tracing, 6 measurements of film
thickness were made. These measurements were taken
250, 500, 750, 1000, 1250, and 1500 µm from the
edge of the die spacer coating. Eighteen measurements
(3 tracings × 6 measurements per tracing) were
obtained for each sample. These 18 measurements
were averaged to obtain 1 film thickness per sample.
A 2-way analysis of variance (ANOVA) was per-
formed to determine the possible significance of any
differences in film thickness with mixing technique or
open bottle time (Sigma Stat, SPSS Inc, Chicago,
Ill.). If a statistical difference was noted within a
group, the Student-Newman-Keuls test with a set
P≤.05 was used as a multiple comparison procedure to
isolate any differences.
Fig. 2. Schematic drawing of top view of slide coated with die spacer. Dotted lines represent
path of stylus across coating during profile trace. Open circles represent locations where film
thickness measurements were made.
Fig. 3. Typical film thickness profiles recorded by profilometer during single trace along die
spacer coating. Profiles are for handshaken and vibrated mixing for die spacer bottles open
for 24 hours.
PSILLAKIS ET AL THE JOURNAL OF PROSTHETIC DENTISTRY
A composite plot of averaged die space film thick-
ness values for all variables with standard deviation bars
was constructed (Sigma Plot, SPSS Inc).
Typical thickness profiles for both handshaken and
vibrated films are provided in Figure 3. Mean values
for each sample were obtained by averaging the thick-
ness both over the 6 points along a single profile trace
and over the 3 tracings per sample (Fig. 2). A plot of
the effects of mixing method and open bottle time on
die space film thickness was made (Fig. 4). For mixing
by hand (shaking), the thickness varied over time from
4.0 to 17.4 µm with an average of 8.7 µm. For mixing
by vibration, film thickness varied over time from 2.4
to 6.6 µm with an average of 5.0 µm.
A 2-way ANOVA was performed to determine the
significance of mixing method and open bottle time.
The larger thickness found for the handshaken over
the vibration group was statistically significant
(P≤.001). To isolate which times differed, the
Student-Newman-Keuls method was used. The film
thickness differences between handshaking and vibra-
tion existed at 4, 8, and 24 hours. The difference in die
spacer thickness versus the time the bottles were left
open was statistically significant (P≤.001). The
Student-Newman-Keuls results showed statistically
different film thicknesses for all time comparisons
except 0 versus 1 hour and 4 versus 8 hours. This
result remained when considering only the samples
within the handshaken group. Therefore, the results
indicate that die spacer thickness increases with open
bottle time when the bottle is handshaken. In contrast,
within the vibration group, no statistical differences in
film thickness existed with evaporation time except for
0 versus 4 hours. Therefore, the results indicate that, if
the die spacer components are mixed by vibration, die
spacer thickness does not increase with the time the
bottle is left open.
The results indicate that evaporation of die spacer
components does lead to larger die spacer film thick-
ness. For handshaken bottles, die spacer thickness
increased by –1%, 142%, 105%, and 329% for bottles
left open for 1, 4, 8, and 24 hours, respectively, com-
pared with bottles open for 0 hours (Fig. 4). This
trend was less dramatic for bottles mixed on the den-
tal vibrator: Die spacer thickness increased 74%,
171%, 111%, and 173% for bottles left open for 1, 4,
8, and 24 hours, respectively, compared with bottles
open for 0 hours (Fig. 4). These values are in the
range of those found in a previous study, in which a
148% increase in film thickness was found with “old”
versus “new” Tru-fit bottles.27In that study, “old”
bottles were simply defined as those previously used in
the clinic; they therefore had been open for some
unknown period. Similarly, Grajower et al23found a
140% increase in Tru-fit thickness for 6-month-old
bottles versus new. It is possible that such large
increases in die spacer thickness with open bottle time
are clinically significant.
Fig. 4. Plot of film thickness versus mixing method and time die spacer bottle was left open.
Bars represent averaged values; vertical lines on bars represent standard deviations.
THE JOURNAL OF PROSTHETIC DENTISTRY PSILLAKIS ET AL
86 VOLUME 85 NUMBER 1
For bottles that are handshaken per the manufac-
turer’s instruction, increases in film thickness occur
when the bottle is open for sometime between 1 and
4 hours. If it is assumed that a bottle is open for sev-
eral minutes per application onto a clinical die,
clinically significant increases may occur after painting
20 or 30 dies.
The clinical consequence of larger die spacer thick-
ness found with open bottle time may be excessive
cement film thickness. With all other variables held
constant, an excessive luting agent film thickness may
lead to lower restoration retention and/or higher lut-
ing agent solubility at the margin.26Conflicting
experimental results exist for crown retention7versus
cement thickness, possibly because of the multitude of
experimental and/or clinical variables as well as differ-
ences in experimental design.
Greater die spacer film thickness with open bottle
time probably is caused by the subsequent higher con-
centration of metal-oxide particles in the die spacer
solution, with more metal oxides being applied per
brush stroke. The higher concentration of adhesive
during evaporation also may contribute to increased
During mixing, die spacer particles that have set-
tled during the open bottle time must be evenly
dispersed to obtain a sufficient quantity of particles, as
well as adhesive, on the brush. Although vigorous
shaking by hand for several minutes is recommended,
such shaking is not always achieved clinically. Mixing
bottles on the dental vibrator was chosen as a consis-
tent technique for obtaining less-than-optimal
mixing. Although thickness tended to increase with
time with vibration, the vibrated thickness values were
consistently lower than those obtained from hand-
shaken samples. Our results suggest that, during
vibration, the die spacer solution is not homogenized;
a significant percentage of particles remain near the
bottom of the bottle, possibly in congregates. The
brush therefore does not capture a sufficient number
of particles to obtain full thickness.
The increase in spacer thickness with open bottle
time for vibration occurs earlier than that for hand-
shaken samples (after 1 hour as opposed to 4 hours)
and seems to remain fairly constant. It is possible that,
even with the evaporation of liquid die spacer compo-
nents with time, agitation with vibration is still not
great enough to disperse the particles as well as shak-
ing by hand.
The variation in applied die spacer thickness per
brush application in the literature is quite large (6.0 to
19.8 µm per layer8,13,15,17,19,23-25,28) and probably is
due to variations in clinical and experimental tech-
nique17as well as differing positions on the dies,17
compositions, brushes, and substrate. The Tru-fit
manufacturer claims that 4 thin coats provide 25 µm
of internal relief,7which averages to 6.3 µm per coat.
The results of this study were within or close to these
numbers. The average film thickness over time was
8.7 µm (range = 4.0-17.4 µm) after handshaking and
5.0 µm (range = 2.4-6.6 µm) after vibration.
For handshaken bottles in this study, 4 applications,
on average, provided a die spacer thickness within the
suggested optimal 25 to 40 µm range.4,11,12,17,22,23-25
Four coats of die spacer from bottles open for 0 or 1
hour provided a thickness of 16 µm, slightly less than
the suggested optimal range. This lower thickness
most likely was due to the wetting properties of die
spacer liquid on smooth glass versus porous die stone.
For bottles open for 24 hours, an excessive film thick-
ness may occur with 3 or more applications.
Variation in the results of this study was low, with aver-
age standard deviations of 1.8 µm (range = 0.8-4.2 µm)
for handshaking and 5.0 µm (range = 2.4-6.6 µm) for
vibration mixing. Standard deviations in previous studies
ranged from 4.5 to 19.9 µm for Tru-fit multicoated
This study had limitations that hinder its direct clin-
ical significance. Namely, it was designed to be a
preliminary study to investigate only 2 of the many
variables associated with die spacer thickness: the
effects of evaporation of volatile liquid components
and the intensity of bottle agitation during die spacer
mixing. To keep the number of variables to a mini-
mum and to attempt to decrease the high spacer film
thickness variability found in other studies, only 1
brand of die spacer and 1 brand of glass slides were
used. Decreasing film thickness variability was critical
to determine statistical differences with mixing and/or
open bottle time. As stated previously, the variation in
film thickness found here was lower than in other stud-
ies. Additional studies that include other variables
should be undertaken.
Several concerns arose regarding the choice of sub-
strate for the die spacer films. Flat, smooth, uniform
surfaces with little or no intersample surface variation
were required, and glass microscope slides were
deemed appropriate. Die spacer was not applied to
stone for the following reasons: First, die stone is
porous and comparatively rough. Applying spacer to
die stone might have affected the film thickness found
with profilometry. We wanted to be sure that varia-
tions found during the profile tracing were due to the
die spacer film and not the roughness of the underly-
ing stone. Moreover, because of the porosity of stone,
some of the die spacer liquid might have entered the
pores; the thickness of die spacer within the pores
might not have been detected with profilometry.
Second, the use of die stone would have introduced
additional variables to the study. The choice of which
brand of stone, the addition of hardener,15and any
manipulation variables incurred during stone mixing
PSILLAKIS ET AL THE JOURNAL OF PROSTHETIC DENTISTRY
and setting would have increased the variability of the
results. Therefore, for this preliminary study, smooth
glass seemed appropriate.
Only 1 die spacer was chosen for this preliminary
study. Most traditional die spacers are somewhat sim-
ilar in composition; they comprise metal-oxide
powders, adhesive, and a volatile organic matrix liq-
uid. Differences in evaporation rates and the effects of
evaporation, if any, are expected with die spacers from
different manufacturers. However, the volatile com-
ponents of all die spacers do evaporate, and it is
expected that this evaporation may affect thickness.
Therefore, for this preliminary study, the use of only
1 die spacer seemed reasonable. Gold Tru-fit was
deemed a good choice because several studies related
to die spacer thickness with Tru-fit exist in the litera-
ture8,13-15,17,23,24,27-29and Tru-fit is commonly used
in clinical settings.7Because positive evaporation time
and mixing agitation effects were found, future stud-
ies should include various brands of stones and die
spacers to enhance the direct clinical significance of
This investigation, the first quantitative, well-
controlled study to report the effects of evaporation
and agitation on die spacer thickness, found that die
spacer thickness increased with open bottle time and
decreased with lower agitation during mixing. These
findings compare well with other relevant results in the
literature; however, variations in thickness values were
lower in this study than in others. Further investiga-
tion is warranted to determine the clinical importance
of the results reported previously.
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Reprint requests to:
DR JASON J. PSILLAKIS
DIVISION OF PROSTHODONTICS
SCHOOL OF DENTAL AND ORAL SURGERY
630 W 168TH ST
NEW YORK, NY 10032
Copyright © 2001 by The Editorial Council of The Journal of Prosthetic
0022-3913/2001/$35.00 + 0. 10/1/113028